Wow - I'm really surprised that this is still going. Here are my thoughts on the various subjects at hand, for whatever they might be worth...
As for parents being in the school, as I've already stated I'm in our school all the time as a volunteer and I'm 100% for it. My DS attended a school for 1/2 a year that did not welcome parents, and I was very uncomfortable with that.
As for parents having lunch with their children, I see nothing wrong with that. Some parents can, some can't. Just as some parents have the time to volunteer, some don't; and some can chaperone field trips, and some can't.
As for parents bringing in a special lunch to share with their child, again I see nothing wrong with that, especially if it's a once in a while kind of thing. What one parent does for their own child is pretty much none of my business.
As for saying that letting a parent bring in a treat for their own child is wrong because some parents can't do that, well I think that's taking it too far. That would be like saying I can't put a treat in my child's lunchbox, or buy my kids LL Bean backpacks because some parents can't afford them, or that I shouldn't volunteer because other parents can't.
As for parents bringing in treats for just a few of their child's friends, I say that's not a good thing. I don't like the exclusionary nature of this. If there are treats for more than your own child, they should be available to all of the classmates. Just like if I send my own child a cupcake in his lunch, that's fine. But just like when I send in treats for his birthday there should be enough for the whole class, then likewise if I bring in a treat at lunch for anyone besides him it should be for everyone.