Should we just let any old thing that slips into common usage to become the norm?

Yes.

Why not spell it "definately"? It's very common and everyone understands it.

I don't think that quite meets the threshold yet, since most people who do that would still agree that it's not correct. However, it's close, and I wouldn't be against recording it as an alternative spelling.

It's a bit tangential, but English spelling is awful anyway, it bears hardly any relationship to the pronunciation, and I think it's great if it evolves to be a bit less unintuitive.

I suppose you probably do accept the existence of American spellings, even if you aren't from there? So the only difference between us is time, and how many people use a variant. Everyone is a descriptivist, some people just also think they should force their opinions on others, which is wrong. ;)