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What you have to watch out for is that some of those crummy directories are set up to be "link magnets" and conduits of links & PR for networks of sites that can sometimes be a bit unsavory.
It depends on how crummy they are. If they're crummy with the slightest suspicion of anything not 100% above board, don't link back.
It'll work OK until Google finally realizes that those 10,000+ links all come from the same frigging website. I wish G would wake up and figure out how to nuke these irrelevant purchased backlinks already. I admit, I've had to subscribe to one of these just to keep up with the Joneses- yuck! Yeah, you are free to buy ad placement anywhere, so technically it's not Spam, but technicalities aside, it's...Spam--pure Spam, in intent and in effect. Your tiny hyperlink text at the bottom of some 10,000-page site is not an ad, don't delude yourself. Any way you slice it, it's just a massive backlink bomb.