Would it make sense to ask your mechanic to work on your car for free? Would you look him in the eye, with a straight face, and tell him that his compensation would be the ability to have his work shown to others as you drive down the street? I have lost track of how many times someone has approached me and asked if they could use my work for free. They always use the same tired excuse too - something along the lines of "I can't pay you, but it would be a good reference..." That is absolutely insulting. That is not a "great opportunity." That is pure exploitation.
This happens way too often to all artists of all mediums - illustrators, graphic designers, photographers, web developers, etc. Every amateur can use some experience, but that does not mean one should give their hard work away for free. Your art is a contribution to society. Being paid allows you to make that contribution to society. If anyone ever tries to pull this crap on you, be firm and stand your ground.