I think it very much depends on the value people place on their images.
Pros who earn a living from their photographs place a very different value on their output than amateurs who take photographs for the enjoyment of doing so. Semi-pros also have a very different outlook.
@Lindsay D articulated it very well by writing how publishing a photo for credit does nothing for the photographer unless they are promoted - however in some instances, the mere attribution
is promotion, depending on the publication and context. At
@Lindsay D 's level, this is not sufficient value for her (and I don't blame her) but it may be sufficient value for someone starting out in the industry - someone who needs the exposure more than the cash value.
The point is, the interpretation of 'free' is subjective - an amateur who gets an image credited to them in a publication may not consider that they had received nothing in return for their image. Someone who has overheads directly associated to their photography would in all likelihood disagree - they have to pay for them somehow.
I don't subscribe to the view that anyone who gives away the fruits of their labour in return for 'something' that has sufficient return on investment for them will impact those who don't, but I do appreciate that we should draw attention to the thought process behind doing so. Discussions like these have really made me think about the value I place on my images and whether I would be prepared to work for 'free' or not. Also, I think that the awareness raised regarding unscrupulous tactics and practices by newspapers and (insert professions here) in soliciting images from people for the promise of exposure, should be a warning to anyone who values their images.
In my own career (non-photographic related), I have had to adapt and change my course to ensure I stayed ahead of the game. If I had sat back and moaned about all the amateurs who were encroaching on my space rather than diversifying and pushing myself to excel despite them, I'd never be where I am today. This is what sets pros (read, people who care enough about their chosen profession) apart from amateurs.
I don't care if people pimp themselves out for free or not. I don't believe that I will be unemployed because a few amateurs will somehow work me out of a job by performing my services for free. I excel at what I do and I will adapt or die.
If a newspaper asked me if they could publish my image for free, I'd be flattered. Would I allow them to do so? No... not unless I got sufficient return on investment, whatever that may mean to me.