In a zone the SB cones in real handy on your bigs to defend the drives. As many guards at higher level have pretty high IS you may need to add a lot to fully defend a drive in m2m.
Actually I think SB doesn't do much against drives. I'd rather have high ID and low SB than both somewhere in between when defending drives.
However, when a guard posts up and uses IS, I'm happy to have SB on my bigs. If he posts up my guard that doesn't have high SB, I just hope he can defend it.
Also IS doesn't have much of an impact when a player drives. Some yes, but not a lot IMO.
So:
ID defends drives
SB defends IS and a bit of drives
So against IS = ID+SB
Against Driving = ID
Against jump shots = OD+SB
If you are playing against someone with defensive bigs (low IS), you generally don't need SB. It's nice to have, but it's much more important against great IS.