I think that if your called into the studio because of who you are, then you're good. The money is good and the experience is always valuable. If you get called because you can mold your voice and imitate the style of other artists, you run the risk of losing yourself and not being able to respond when it's your time to be you. Our voices are each completely unique, and the only chance we have to stand out from the crowd is to preserve that individuality.
Definitely... as a vocalist, you have to define a certain characteristic or element that is unique to you. By focusing on that, you fine tune and polish who you are. Finding your own "niche" is a huge key to this business of music. You want people to hire you because they know what they will get, and by delivering, you only increase your clientel.
I agree with Ray, although I can imitate the breathy little girl vocals that are so popular, I will never disconnect from those rich alto ranges that are really me. I find that lately I am getting work because of them.
I spent many years doing corporate cover gigs and getting paid quite well for it. I believe it helped to mold and identify who I am AND who I am not. You are who you are. Embracing that is the key to success and balance.
Personally, I use gig money to support my artistic endeavors.