ok I researched this further.
Mariah has had the vocal nodules for a while, but they are a blessing and a curse because she had them before she became famous and they're actually the reason she can sing in the whistle register. She might lose that ability if she has them removed. The deterioration of her voice is also probably due to the fact that she has the nodules and has continued to sing, because the way she sings only makes them worse over the years.
Julie Andrews got surgery on hers, and she lost her singing voice. She now has a 1 octave range. That probably scares Mariah enough to shy away from any surgery. It's really sad because Julie was a great singer and now she can't sing at all pretty much. Imagine if Mariah couldn't sing at all. That would be so sad.
As long as Mariah has enough vocal rest, the nodules don't affect her singing that much. That is the typical remedy for someone with the problem, rather than getting surgery. That's why she still sounds good in the studio, because she has time to rest and perfect it and everything. (pro-tools can't fix the problems she has live, which are mostly her voice cracking or sounding weak at inopportune times) However, during promotion, Mariah doesn't really have a chance to go on long periods of vocal rest, and her performances suffer because of it. As a result, she has 2 options: lip-synch the less important performances or give it a go and try her best. I think she still sounds pretty damn good. Even when she's on tour, she sounds a lot better, because she usually schedules a couple of days of rest in between each date for vocal rest.
sources:
http://rds.yahoo.com/_ylt=A0geu.SR5h...al_fold_nodule
http://www.exposay.com/mariah-wants-...voice/v/16296/
15 years of following Mariah