Well said, Chris.
Does CLA need a mastering engineer? Or the other legends of mixing? A lot less than you or I in regards to fixing issues, that's for sure. A big part of mastering is giving what the song needs, which is different for every song, and the pros generally put together a mix that doesn't need much. A less experienced mixer with frequency response biases (maybe he is a bass-head) and a less accurate monitoring system (maybe his speakers are too dull, so he mixed too bright to compensate) creates a lot of issues, and mastering serves as a quality control check.
CLA and others likely need that a lot less. But, like Chris said, somebody needs to bring the mixes up to level, make sure all the mixes are the same level, make sure all the mixes match each other in tonality, make sure the micro-dynamics are controlled, and make sure that the macro-dynamics provide impact. Not to mention track ordering, applying appropriate fades and silence between tracks, turning the entire album into one audio file with track markers for CD, quality control check for clicks and pops, and delivering the content to the CD replicator.
And, exactly like Chris said, if one person is both mixing and mastering a project, it's absolutely preferred to fix the problem in the mix. If the bass guitar is 1dB too tubby at 100 Hz, mastering can only bring down all instruments at 100 Hz, not specifically the bass guitar. It's a no-brainer to fix it in the mix.
A good mixing engineer doesn't make a half-decent mix and expect mastering to fix it. A good mixing engineer makes the song sound as perfect as he possibly can, and as complete as he can. Though it's definitely best practice to leave master bus compression and limiting off the track, since the mastering engineer can't restore dynamics if the mixing engineer goes too far. In those cases, a decent mastering engineer has to ask the mixing engineer to turn off the limiter and send it again. If that can't happen, the mastering engineer just has to turn the loud track down to match the others, meaning it had the life smashed out of it for no reason. I've seen it happen, and it's not pretty.