That said, a good speaker system (especially multi-channel) is basically requisite for movie-watching or sharing music with another person. For example I watched Harry Potter 3 last night on my big-rig surround system, and wouldn'tve had it any other way - none of my headphones come close to reproducing the same experience (keep the TV and everything else equal, and just switch to headphones - it's still no good imho).
I think the points on detail retrieval with headphones are well taken too - but that isn't to say that good speakers can't be fairly detailed in their own right. But it is much different, to say the least. Sometimes I just want to hear how the music would sound through a pair of Grados, and other times I'd prefer to listen to it DSP'd onto a 8.2 speaker array with big subwoofers and feel my chair shake when the bass kicks. There's no "technical" reason for either preference, it's just whatever mood strikes me. Unfortunately the later is far less practical (the big-rig is capable of getting loud enough to be heard from the street, I'm sure my neighbors would not appreciate that during the night)). Still, during the day, when that's a non-concern, and I have the house to myself (except for my dog (she really hates the subwoofers and the automatic relays in most amplifiers)), it's kind of a coin-flip in terms of what I'd like at a given moment. So I appreciate having both.