Juliechan, I agree to an large extent; most people don't think too much about what they are saying and just copy what they hear, but language has been evolving organically logically ever since it ever began. There are some elements that I would say have slipped past the censors so to speak, but overall language is based simply on what makes sense, e.g. you don't say "I will go to Tokyo yesterday".because of the inherent meanings of the words, and if that sentence was acceptable the meanings of the words themselves would have to change.
Ootmaster, I'm not sure which sentence you are referring to, but what I find strange is, in "I'll be going to see a movie tomorrow", why would you not just say "I'm going to see a movie tomorrow?" I don't understand what is trying to be described in "I'll be going to see a movie" or any situation you would use it in. Again, perhaps it's an American English expression but to me it doesn't make sense, and there are much easier, more normal and straightforward alternatives. I can almost get my head around "I'll be going in July, to attend a conference," but in this example I have to separate the clauses with a comma, or put in "in order to" for it to make sense to me. In "I'll be going to attend a conference" it sounds like one run-on sentence without an appropriate separation.