Thanks, and I think some of the confusion is that there are two different arguments that are being mixed into one.
The first is as I stated, there is one continuity, but it's not perfect. So by using Time Travel, they have an opportunity to fix it up so that it makes more sense and ends any confusion that has occurred since each of the films were released.
The second argument, which is mostly based on speculation, is that whenever you use Time Travel, which in this case I don't believe is being used a lazy device to erase anything, there is always some kind of ripple effect. And as I've mentioned, it all depends on what time period they go back to or forward to. The fact that Patrick Stewart is returning suggests that it will include scenes from before the end of X3. And the speculation is that by using Time Travel, and having people from the future go to the past, they will change things that could lead to other changes, such as Xavier never dieing, that way they can bring him back for a sequel if they ever choose that. It wouldn't be retconning because it would work in the realm of Time Travel. It's not like in SM3 where they suddenly said "what you think happened didn't really happen the way we said. Here's the real story." That was a retcon. This is more like "What happened still happened, but someone went back to a time before it happened, and prevented it from happening."
Man, this is why they always say don't talk about time travel because you're just going to confuse everybody.