Please post evidence or provide logic reasoning for why unilaterals should balance out the legs more than correctly performed - which means balanced - bilateral exercises.
During a squat for example, the outer quad(vastus lateralis) is recruited more than the inner quad(VMO). This will eventually lead to an imbalance, which if ignored can result in the knee 'caving in' b/c of the lack of strength of the VMO which may lead to injury. This is also why the VMO is heavily rehabbed in ACL injuries, to prevent further injury and 'caving in' of the knee. This is coming from experience.
And please explain why a powerlifter should do unilateral work when everything he does in his sport is bilateral in nature and will usually require a coordinated, well-balanced effort of both his left and right body side.
Injury prevention/Muscle Balance/Recovery/Variety/Wanting to have a longer, healthier career as a powerlifter.
Also as Andrew has pointed out, eveyone has a dominant limb and that limb shall stay dominant, so it's a good idea to keep the race a close one.
And don't just say "you can always do both", because only an idiot would bring forward such an argument,....And I don't think that you are an idiot,
You are contradicting yourself here. I say you can because you can. Don't put yourself in a box.
since it means that he does not understand that everything that is useless is in fact also detrimental because it means that valuable training time as well as physical resources are wasted which could otherwise be used for productive things that actually do something to help ones progress.
I'm sure you didn't assume this but not alot of energy/time has to be given towards single leg work for powerlifters, bodybuilders or olympic lifters. You wouldn't have to dedicate a whole session to single leg movements, just enough to further progress, strengthen weak links and to stay healthy.