My feeling is on the contrary that the set of people having a real knowledge of gcc (or at least of substantial parts of it [*]) is much bigger than the set of reviewers allowed to say OK.That's certainly true, but there's a big difference between having real knowledge of gcc and having enough real knowledge to approve a patch.
It's also a matter of time. I wouldn't trust myself with the ability to review patches in broader areas, because I wouldn't have enough time to do it properly.
Paolo
