Why sign it?
For legal reasons, photographers need the protection of a model release, especially when shooting photos with nudity. They need proof you’re over 18. And the model release protects them from lawsuits if they use the pictures in the way the release states — even if you don’t like it.
For you, the protection comes in the form of what rights the photographer does and doesn’t have with your image. If the photographer is using your pictures in a way not clearly stated on the release, you could legally compel them to stop.
Most model releases, at least with print/digital still photos, is boilerplate. It usually gives the model the right to use the photos for their own self-promotion but not to sell. It almost always gives the photographer the right to use the photos in their own portfolio, to sell or to use in any other way they need to promote their business (e.g.,advertisements, Web site).
If you don’t like any of the things contained in the model release, you need to change the contract right there and make sure the photographer signs. Discussing this well before the shoot will keep both of you from wasting time if you can’t come to an agreement.
But you must change the model release if you don’t like the terms. You can’t just make a verbal complaint and expect the photographer to follow your wishes.