Actually professionals do shoot for "free" in a sense. In the industry it's called TFP (time for print) where a model will give her time for free in exchange for photos for the model's portfolio. That way a photographer can practice new lighting techniques, poses, processes, or ideas a photographer just wants to get out of their head.
In the end both model and photographer will have new work to keep their respective portfolios fresh. These photos are for portfolio use only as well as for promoting their respective services. Neither are allowed to use these photos for commercial purposes (selling prints, use in advertisements or publications). But a deal can be agreed upon later if decided.
Here in the Philippines these are also called test shoots, portfolio shoots, or x-deals. And instead of TFP some now use the terms TFCD or TFDVD.
Some of my TFP work...
