consider the following scenario, in low light conditions.
f5.6 @ 1/60 + IS vs f2.8 @ 1/120
(numbers are randomly plucked from thin air)
u can stop motion (acceptably) with the f2.8 at 1/120, but at 1/60 likely to have motion blur of your subject. IS is good for handholding, but it cannot replace faster shutter speeds especially when shooting moving targets. nice to have, but not a must have.