by close micing, you essentially take the room out the picture... stick your ear right up to teh grill and listen. put a mic back where the guitarist usually stands and there will be a major "bloom" difference. i like a combination of both, but it also depends on the sound you are going for.
also WHERE you put a mic up close, how its oriented on the grille, centers or tot he side, on axis or off axis. its also why ribbons [with a F8 pattern] are helpful because they do caputre the backside room of the amp.
sticking it up close is usually for isolation to OTHER sounds in the room.