I do, but my answers get voted/thumbed down anyway, or I get a nasty email from someone who suggests I didn't even answer the question, let alone provided the right source (twice from the asker!)
When someone asks a question of a detailed fact, I'll source what I can. But sometimes, the questions are vague, so I give a broad answer or opinion. I've come to the belief that many times, people don't want sources, they just want answers - this is typical in the computer or homework sections where the askers are too lazy to read the source, and it happens here occasionally, too.
Also, some topics relating to history are notoriously controversial or devoid of source information (like taekwondo history). For every one book that says one thing, there's two more out there that says something contradictory.
I honestly think some people would not write what they do if they also had to provide a source. It's clear that some have no idea what they're talking about. So, just take it all with a grain of salt. Consider it all opinion, even if sourced.
It would be nice if Yahoo encouraged sourcing (say, by giving extra points for sourced questions), but this will probably encourage inane or spam sourcing, I think.
EDIT: You know what else? Sometimes, people will ask a question: it's bad enough that the question's been asked and answered before (like, "how to stretch"). When I first started answering, I sourced my answers in this regard. But people still ask the same questions over and again, which tells me:
..either they researched the question before they asked it, and came to the conclusion that none of the previous answers (including my and anyone else's sources) are adequate; OR
...they're too lazy to research the question in the first place.
In either case, I (1) won't answer the question, or (2) if I'm in the mood to answer I refuse to source it.
If they didn't like the previous answers I gave, there's nothing I can add again that might be more helpful. If they are too lazy to research to see if the question's been asked, I'm not going to waste my time giving them any more information. In fact, I've stopped answering repeat questions: why should I keep writing the same answers over and again?