JOEL - I am very active in the church and very much enjoy the research I am compeled to do to answer the many questions people ask me.
You are correct in that much of what we can learn can come through both the scriptures and from personal revelation from God. But perhaps I could make a couple of points relating to involvement in the church organization itself.

The scriptures tell us:
"And behold, I tell you these things that ye may learn wisdom; that ye may learn that when ye are in the service of your fellow beings ye are only in the service of your God." (Mosiah 2:17)
We can give some service on our own without needing to belong to an organization, but the church is the best source for finding out what kinds of service are needed and it is the best vehicle for organizing that service for the best effect. The home teaching and visiting teaching programs come to mind where people can be immediately made aware of what is needed and can summon much help at a moments notice to render that service for both church members and the community.

The Church membership is also a source of comfort and support for all members and the interaction between members serves to strengthen each others' testimonies of the gospel. Perhaps you have heard of the following story:

Long ago a church leader came to visit a member who complained that there was no real reason for him to attend church or get involved. The leader noticed the member's warm fireplace where the pieces of wood had burned brightly, with the embers still glowing and giving off heat. He then proceded to take in hand some brass tongs and removed one of the hot glowing embers. That ember slowly turned pale in light and turn black. No longer would it glow. No longer would it warm. He then demonstrated that by returning the black, cold ember to the bed of living coals, the dark ember would begin to glow and brighten and warm again. He concluded, “People are somewhat like the coals of a fire. Should they absent themselves from the warmth and spirit of the active church membership, they will not contribute to the whole, but in their isolation will be changed. As with the embers removed from the heat of the fire, as they distance themselves from the intensity of the spirit generated by the active membership, they will lose that warmth and spirit.

The New Testament tells us:
"For where two or three are gathered together in my name, there am I in the midst of them."(Matt 18:20)

"And let us consider one another to provoke unto love and to good works: Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching." (Heb 10:24-25)

Our testimonies and spirits are strengthened by our involvement with other church members. We can more effectively share our God-given talents with others and it is within the church that we are able to learn from prophets of God how to apply the gospel to our lives in these latter-days and how to interpret scripture.
For example, you refered to the scripture where Jesus says the "kingdom of God is within you". (Luke 17:21) Actually a more correct translation of that scripture is:
"the kingdom of God has already come unto you." (Joseph Smith Translation)
Jesus is refering here to the fact that now that He is on the earth, His kingdom(His church) is on the earth.

DAVID - Mormons believe that "cheap grace" doesn't neccessarily save the soul unto eternal salvation, as many 'other' denominations tightly linked with Pauline thought might have the multitudes believe, am I right? Instead we go from 'grace to grace,' delighting in good works all the way home. Jesus Christ, therefore, is an example and archetype of what all men can strive to be, and will one day accomplish. I ask you: did Jesus feel the need for outward church? Actually he criticized those Pharisees of it. He was, in modern-day terms, a radical rejecter of the organized hierarchy of the ancient Hebrews. The Church, he said, is within; it is the kingdom of the soul. Only by finding this inner kingdom can man truly and effectually minister unto his fellow man; and to suppose that such a task can only be done at or from a church or temple is outright absurd. It is to be done in every waking moment of life, in or out of our materialistic organizations.

Your coal and ember story only applies to those still in need of taking the first step towards salvation, which is repentance and baptism. Stagnant dependency on the church for spiritual meaning in life will keep us always in that state of reliance on others, when all the mysteries of the kingdom are within our very spirits. Reverting back to the old ways isn't progression; it's walking backwards. If we are to become as Jesus, we must do even what he did.

"The kingdom of God cometh not with observation: neither shall they say, Lo here! or, lo there! for, behold, the Kingdom of God is within you." (Luke 17:20-21)

Or perhaps Paul's rebuke of the church (external congregation) of Corinth might also suffice the advancement of this message:

"Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you? If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temples ye are." (1 Cor 3:16-17)

JOEL - Jesus must have felt the need for a church. In his letter to the Ephesians Paul said of Christ:

"And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive;"(Eph 4:11-14)

Jesus set up His church with all those officers for the purpose of perfecting the church members and edifying the body(church) of Christ at that time. He knew it was important so that all would be in a unity of faith and not be confused by outside doctrines of men.
Then speaking to His head apostle Peter He said:

"And Jesus answered and said unto him, Blessed art thou, Simon Bar-jona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.
And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it.
And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven." (Matt 16:17-19)

Jesus gave Peter the keys and authority to lead His church through revelation from God.
Jesus only rejected the organized hierarchy of the ancient Hebrews, because they were still following the old laws of Moses which He came to fulfill.(Matt 5:17)
I don't disregard your scripture references. I agree with them as well, but you need to consider all scriptures in the Bible concerning what God would have us do in His kingdom.
I agree when you say that finding the inner kingdom should not be done only from a church or temple. But it is important to have a healthy combitnation of both the inner and outer kingdoms.