I've noticed a lot of providers in my area  advertising that they're well-tanned.  So, I'm guessing it's because men generally tend to prefer that?  

For the record, I definitely go for the women with more natural skin tone.

Tan or Alabaster skin, both are very attractive. It is all about the overall appearance and attitude that a lady has.

b-

A tan makes one appear thinner and more toned.  I prefer blondes to be tanned but I like my brunettes a bit fair.

...have some tan lines.  Love dark nipples on light skinned boobies.  Nothing worse than an all over tan, where the skin is tough and dry.