This argument is used a lot and I don't really understand it. If you are really confident that Yahoo's stock will triple but you like Google better you could just buy shares (or options) to hedge that.
On a purely economic basis you probably are right. But employee options are granted, not purchased, and I suspect you work harder if you have skin in the game.
Also, from a self-actualization perspective it may be preferable to by VP of Worldwide Widgets at Yahoo than Director of EMEA Widgets at Google.
