Suppose you pay your top guys $100k, and a candidate comes along with about the same level of skill and experience that you really want to hire. But he won't join for less than $120k. At this point you either have to pass, or you have to bump everyone else's salary. If you have ten guys, instead of paying extra $20k, you suddenly have to pay extra $220k - a massive difference. Same goes for stock options.

I haven't heard a good argument in favor of hiding stock option percentages - I can't think of any other than concealing the fact that your share is negligible.