I stopped doing free portfolio shoots a while back because I always ended up running into someone who wasn't willing to live up to their end of the bargain. I did work with some really wonderful photographers who helped me get good images for my portfolio but there were many others that not only did not give me copies of my images, they never kept in contact, nor returned my emails or phone calls.
I can't tell you how many times I've had to track down photographers. And when I did, they often only gave me one or two images, and they weren't even high res, so I could not print them for my book, which is the whole reason I wanted to work with them in the first place (and they were very aware of what I needed before even shooting together).
Since then I have found much more success with simply paying for shoots in order to update my book. Money talks and I can be a bit more in control of what I want and how I need the shoot to go. I know that I put paying projects in front of non-paying ones. Of course if a photographer contacts me and likes my look and wants to do a shoot for trade, I will take them up on the offer but only if I like their work and it is something that I think would benefit my portfolio.
But as of right now, you've gotta pay to play. =)