I just read an article in which the author says: Don't Kill Hitler!
He lists a number of valid reasons. They started me thinking about the same thing.
My list would be shorter. It can be summed up like this: You can't change the timeline.
The way I understand it, it's like this: even if we could travel back in time (and I believe Einstein once said that it's impossible), we couldn't change our own timeline. We could leave it and travel to another one (that is, naturally, if it were possible) and end up being stuck in that timeline/event horizon, but even if you could return to your own timeline, that wouldn't have changed. You will have created a new 'dimension' where Hitler died (when? as a child? early in his career?), but that's all.
Also, if you want to believe the Doctor, certain things are fixed points in time and can't be changed without dire consequences. Others - you can just keep your fingers crossed that your little 'change' is 'unimportant' enough to pass 'the laws of time' or whatever. Fortunately, I think my little life is insignificant enough to fall under the latter heading. So, if the Doctor existed and was right, maybe I could change a few things. If he would be willing to bring me in the Tardis and stand aside to let me change time. That is if I understand 'the timey-wimey stuff' correctly.