The contrast between free internet forums like Yahoo! Answers and the like, or paid legal research as I mentioned in my previous post, is the dichotomy between two things: reliability and ease of use. Paid legal research is very reliable, and very hard to use. Free legal forums are very easy to use, but you're giving up the farm in terms of reliability (see also, Wikipedia).
They can be useful, but only if you know what you're looking for, you know what you're asking, and you know how to vet your answers. Since anybody can post on a forum, the only way that it's useful is if you can sort that information out (provided you get it, as many questions don't get answered on those fora). The funny thing is, of course, that once you get to the point where you can sort that information out effectively, you probably know enough about the subject where you don't need to be visiting a forum to get your answers.