Spec work: a necessary evil or a career killer?

Since school has ended, I have been busy filling my day planner with internet searches, compiling a publishing house mailing list, and creating custom mailouts. All of this work done in search for jobs that will pay the big bucks $$$. You'll notice that there are a lot of job postings out there that are essentially looking for free illustration work. Even though they might sell you on the fact that this work will buy you fame and acclaim (ah yes... hard not to fall for that one), but this kind of exposure might not be what you want. In fact, composing work for free lowers your art's worth in the future. So where do we draw the line? Do we simply say no on principle? And does this also apply to creating custom illustration samples that will appeal to the specific publishing houses? Well I'm not one to simplify things into black and white categories. It ultimately comes down to your own set of beliefs. I'm sure it can't hurt to customize your portfolio depending on the job. I just wish that all portfolio work wasn't creating simply on spec. Wouldn't it be nice if we got paid for everything we did? You don't see doctors performing operations for free, or engineers constructing bridges for free.... I'm just say'n :P