You use a multiband when mastering 'cos you might want real hard compression on the lows, for example, and really soft, or no compression on the high-mids, or whatever you like. Multi-band compression tends to sound more natural, because the whole mix doesn't pump just from an extra loud kick or something, just the lows.
theosd - If you haven't got a sound you like yet, i'd love to have a crack at mastering them for you! Let me know.