I don't know exactly why it is or how it works, but for some reason some weapons have instant mode change, like Flambard or every throwing weapon out there, some though, most of them in fact have a small delay when changing modes.
I still think that Throwing lance is one of the more broken weapons in game. Gah I recently used it with no wpf and it still had good accuracy. My suggestion would be not to actually nerf it (well maybe a bit), but add that small delay for switching modes, and maybe even buff the melee speed a bit if it works.
Regarding other throwing - some weapons like throwing axes shouldn't have it, since they are held like in melee mode already.
PS: I'd opt to have no requirement for any throwing whatsoever, and reduce the stat level of all of them a bit, since the 0 diff lances have proved to be awful without PT anyway (While being good with PT and wpf) but that might be too drastic of a change for you guys to accept ^^