Less mass to rotate, one less chain and 3 sprockets to stop worrying about, a few pounds off the weight of the car and a very slight increase in engine vibration at high revs. I knew when I was approaching the rev limiter when my balance shafts weren't rotating but with them fitted the first indication I get is the rev limiter cutting in...very smooth. But the increase in vibs isn't the annoying type, it feels like the engine is actually working for its living...
I have been told than the slanted Saab blocks in the C900 and early 9000's were subject to vibrations inherant to their design which caused them to be unreliable at higher RPM.
It's possible that these ballance shafts are to, at least in part, avert what may be destructive vibrations.
Many people have removed them, but few of those people have pushed the Saab engine past 7K. I suppose few will do so with ballance shafts or not. But it's worth noting that on C900's, which did not originally come with ballance shafts, even professional racers don't like to push much past 6,500-7,000 and in the interest of being cautious it might be wise to do the same should you remove them.
Then again, at very high RPM the ballance shaft chain may be too weak to function and could snap.
Just making sure that people are aware of possible dangers, even if only possible and not concrete.