Also practice/practise - a doctor's practice / practise the piano.

I don't think it's a waste of time, but language is fluid. Sometimes it changes very rapidly and sometimes slowly, but it's never static. Since the US uses a single spelling for both (and most of the world defaults to using US English) my guess is that the distinction will vanish eventually.

I'm slowly learning not to care about these distinctions when it comes to English, as long as the meaning is clear. The closer you look, the fuzzier language becomes - even in the UK or US alone, there are vast differences in the way it's used.