I agree with questdriven. It is a respect thing to dress nice for church. True enough, attendance is more important than dress, however, wearing baggy shorts with your underwear hanging out or ripped jeans that are dirty when you have better clothes in the closet is simply a I don't care or respect attitude. The homeless man coming in unkempt is a different story however and he should be welcomed and given a seat up front.
I dress casually nice for church. Nice slacks and shirt and shoes, but usually forgo the tie or jacket. My Pastor wears blue jeans and I find that impressive. Less plastic and more realism for his position. A 600 dollar suit is overkill for church and speaks of vanity moreso than the Word.
Dressing nice for church isn't about being accepted, nor is it about proving anything. It's just a common respect thing to myself and to others, in general and for no specific reason other than just making a little effort to just dress a little better than staying at home. As if for instance you were going to go out to eat at a sit down restaurant or similar. I don't dress my best or anything for church like as if I were going to get married or to so someones funeral or some such event, but to drag a comb through my hair and put on some decent slacks and shirt isn't a big deal.
This thread comes up every once in awhile, and it seems like some people are a little sensitive to dressing up the slightest bit to attend church and it's baffling to me a little because I'm not sure why it would bother them. It certainly wouldn't impress God, who I think would think that attendance at all would mean more to Him than anything else. But in life we do different things and go different places and generally dress appropriately for where we are going to. The beach, out to eat, skiing, a day at the monster truck rally, a picnic, a walk in the woods, church, a funeral or whatnot, and we dress for the conditions of what we expect and where we go. Not a big deal at all I think. Just to not roll out of bed and put on yesterdays clothes before going is more of respect to oneself than anything to do with the destination.