In a punchy crit with constant accelerations, absolutely. For sustained efforts like time trials, it makes essentially no difference. Once you're up to speed, it's all aerodynamics, gravity and rolling resistance. Ondřej Sosenka actually used a weighted back wheel for his successful hour record ride, with the logic that it would act as a flywheel in the later parts of the hour and smooth out his ragged pedalling. Of course he did test positive for methamphetamine in 2008, which may have had something to do with breaking the hour record.
It's interesting you mention rolling resistance because in mountain biking, cross country anyway, where there will be climbing on loose surfaces, you have to trade off rolling resistance of the rear tire against traction.
If you over-inflate your tires, you'll have less traction.
There's also the fact that during a tough climb on a trail with obstacles (roots rocks) you can't just power your way up, you have to maintain constant awareness of where the contact patch of your rear wheel is at all times, and modulate the power such that you deliver the most power where you have the most traction.
Fail to do that on a rock or root and you'll blow the climb.
If you can shave off 500g in your wheelset, it makes much more difference than shaving 500g off the frame or other 'static' component.