It allows value to accumulate without taking tax consequences. Unless it is ruled illegal, in which case everyone except Uncle Sam loses. So it has been good for major Microsoft investors (particularly Bill Gates), but is legally risky.

But it is fair to point out that Microsoft is in a form of business where they need to make large and risky long-term investments. This limits them to lines of business with very high returns, and gives them a legitimate reason to have large reserves. (If not as absurdly large as the ones that they currently have...)

Cheers,
Ben