Quality Vs quantity in general. Despite the space it takes up, its better (read as...cheaper) for mobo makers to use a large number of crapply implemented phases than a fewer number of really well designed/specced phases.
A good single phase costs more than 2 poor ones.
Thats not to say the Asus is crap, but most mobo makers are marketing gear as "more is better": 8 or "pseudo-12" phase CPU voltage derivation, masses of heatpipes look good, but the same job can be done with less- it just doesnt look so "visually impressive"
Intel design spec says that no less than 4 phases can be used on a C2D mobo.
examples:
DFI Blood Iron: per phase: 3FETs, 1 choke, 1 pre-phase cap, 2 post phase caps (I think)
P5K3: per phase: xxx FETs (under heatpipes) 1 choke, 1.5 caps per phase and 1 control IC of some kind
Giga P35 boards: per phase: 3 FETs, 1 choke, 1.66 caps per phase (a bit odd) (the DQ6 has 2 chokes per phase)