It depends completely on the cavalry man, there are very few horsemen I respect as players (as people is a different matter) because they just cannot play. It's the same issue as archers, if you don't know what you're doing, then don't shoot into melee. If you can't ride a horse properly, don't ride through melee.
Sure, even experienced riders will make mistakes or won't judge irratic sudden movements from allies and will accidentally kill, and sure, that'll piss the hell out of the ally who died but it's easier forgiven if for example, you yourself Torben, teamkilled me, I'd swear and call you names under my breath but I would forgive you quick enough just remembering the number of times you've helped me out.
As I say, there are very few cav that I would trust to help me, same as there are few archers/xbowmen who I would trust to do the same. It comes down to control, judgement and generally just the players skill against the ratio of "helping that guy" to "taking the kill".
I know it's a team game, but is there a point in attacking a peasant vs a knight when you know that the peasant doesn't have a hope in hell of surviving? No, because the Knight can dispatch the peasant and get on, you're more useful to go somewhere else and help.
Hope that makes sense, it's rather a grey area, but certainly, if you have the ability to do something, do it, if you don't, don't.