i look at recording (and mixing) as similar to building a house. you have to start with the foundation, and in the case of modern (rock/pop/country) music, that's drums and bass. the guitars/keys/etc provide the walls. the vocals are the roof that sits on top.
the vocals are what sells the song. you have to make sure they work within the context of all of the instruments and the mix.....and when you're talking about the world of multitracking and overdubbing, the only way to ensure that is to record the vocals last. otherwise you end up in a world of hurt if you record vocals first, and then start laying down organ, mandolin and guitar tracks over top of it.
sometimes when i'm mapping out the rough sketch of the song, i'll start with a DI'd guitar and scratch vocals to a click track. that helps with the arrangement, etc., but rarely do those vocals get kept. and if you're recording a band tracking all in one room and have vocal mics open, you're gonna get an inordinate amount of drum and guitar bleed into the vocal track.....if you've ever tried to mix a vocal that's littered with cymbal and guitar fuzz, you'll know how much easier it is to work with a separately recorded vocal.
at times it does feel kinda like karaoke. but if you can get a compelling enough sounding cue mix in the cans, you can elicit a HELL of a performance from a decent singer.
and sometimes, some singers can't pull it off and work a LOT better playing/singing together since that's what they're used to doing. do whatever works best for you.
cheers,
wade