ElFenix, I couldn't agree more. I hate to see that even Obama does it, but it seems pretty hard to resist...there is really no incentive to stay on topic, and all the incentive in the world to turn the whole thing into a free campaign commercial. This issue was addressed well, I thought, by The West Wing TV show of all things. Rather than having a moderator ask questions back and forth and not really pay attention to the answers, each candidate could respond to the question, then his or her opponent could ask a follow-up question which the candidate would then answer. Of course they could ignore the question and continue with the campaigning, but it looks a lot more obvious when you're evading a direct question by your opponent.
My biggest problem with the Democratic debate was that it was hard for both Obama and Clinton get to past how awesome they apparently think the other one is. Which is great, but they are SUPPOSED to be differentiating themselves, not presiding over vigerous agreement. Not only does it make it look like a choice is totally pointless, but it brings up the question of who can win against the Republicans...because I'm sure the Republicans will burn the house down to win the election, just like they always do. If the focus was more candidate vs candidate, it would better encourage the kind of real debate these campaigns sorely need.
I think Kerry proved in 2004 that this works fairly well. He lost the election (for a number of valid reasons), but I think it was as close as it was in large part because Kerry just wiped the floor with Bush in the debates. Kerry did a great job showing that he was presidential, and that Bush was not, mostly because he actually DEBATED a lot of what Bush said rather than just jumping into his pre-scripted talking points at the first opportunity. Bush, who clearly was NOT prepared for anything other than pre-scripted talking points, looked like a tool. Had the first debate, especially, been held closer to the election, I think we'd have Kerry as President right now...in spite of all his faults and shortcomings. And just imagine a real debate that was full of that sort of actual debating, from both candidates...it would be a HUGE improvement, I think.