An op-ed piece in the NY Times by Richard Kahlenberg titled "Elite Colleges, or Colleges for the Elite?" suggests that legacy preferences may be illegal, and that traditional reasons for admitting legacy candidates (e.g., higher alumni giving) aren't supported by statistics.
From the stats I've seen, being a legacy isn't much of an advantage at most elite schools, and legacy status often results in a "courtesy waitlist" that, of course, never turns into an acceptance.
Of course, if your parent (or grandparent) is able to make an enormous contribution, that's a different story. Kahlenberg might find that even more problematic, but institutional survival is still Job One at colleges and universities.