congrats on dragging this back on topic
i saw somewhere the difference for someone like kubica is that with kers he has less than 10 kilo of ballast to play with which gives a lot less scope for tuning weight distribution compared to the non kers cars which can have up to 50 kilos to play with. or to give an example of how kers puts the preassure on drives weight, anthony davidson plus a mercedes kers system weighs the same as rubens barrichello without kers.
the other problem of course with kers is that the weight is in one or a few fairly fixed shape pieces rather than being able to shape it into as many pieces and different shapes as you require. some teams have mounted it below the fuel tank which reduces capacity or raises wieght distribution, BMW have split it to fit each side of the fuel tank in the side pods but are encountering packaging compramises and have suffered from the effect of its raised position.
when the power produced is raised over the next few seasons it will become pretty well impossible not to use it and be competative which means they will have to establish a weight limit that doesn't penalise drives excessively for being larger than others though of course it's only relatively recently that the weight limit has included the driver, prost certainly always used to point out to team owners that he bought a substantial weight saving compared to drivers like mansel, at times as much as 30 kilos !
it must be said that one area of this regs that have worked perfectly is the kers power limit, it has been set at a level that overall there is very little advantage in using it which allows teams not too whilst at the same time encourages some teams to use it and gain experience with it before the power is increased. it will be interesting if the teams that are using it can put that experience to use when the power level increases (providing they are still in F1 )