They are one more traffic source, the more the better.
The traffic is better filtered so the surfer often will find what they are looking for a buy something. Also link lists for the most part don't skim their listings so the traffic tends to be of a higher quality.
Also the listings last far longer then most TGPs which often drop you after a very short time.
Some places are a hybrid of a TGP and link list so the lines can get blurred.
Personally I look at a set of pics that I buy as a few TGP galleries and a free site. The more things that you can build with a set of pics the more chance of a sale and the less the cost per promotional piece.