In short: No
You might be able to compile paranoia (used in CDex) in OS X, but that ripper isn't any better than the one in iTunes.
As for why someone might want to use another ripper:
This has nothing do do with AAC or MP3 or whatever encoder. This has do do with the ripping of the CD itself (ie. extracing the audio data), and some apps are just better than others when it comes to this, at least if the CD is scratched or damaged in any way.
But, generally, as long as the CD isn't full of scratches, most rippers will do a fine enough job.