I don't think they're stupid, per-se. However, each has problems.
Typing on most (all?) mp3 players is insanely difficult due to limited inputs. If I have to do anything more than changing a single file name, I don't bother doing it at all. The idea of writing code on one makes me shudder.
I am not aware of a Rockbox-capable player that has enough beef to encode movies at a speed that would complete even the shortest of movies in anything that could even approximate a decent time. In fact, I suspect many players' batteries would be drained by the process even while plugged in, before it completed.
PowerPoint presentations has a mix of the two problems. It's complicated enough that I'd not want to even try it on my player, and I bet it'd hit the processor pretty hard.