Clients want work for free? If you comply, you’ll be working on spec. And, as I say in my guide to setting consulting fees, you don’t want to be working on spec.
If you’ve got people asking you to do work for free, ask yourself why going along with it would help your case. If clients aren’t certain you can do work worth buying, why would it help your cause to do the work for free? It just confirms their suspicions!