I agree with Shep, but in the opposite order.
1) When each one of the surviving players thinks that he can't win anymore. Or believes that trying to solo is too risky and could result in a defeat or in a someone else's solo. So since a draw is basically a victory because noone did better than you and many did worse, while defeats and survivals are always defeats, voting a draw could be the best bet you have.
2) When a game gets screwed by CDs and players don't agree on Cancel, Drawing can be a fair enough solution.
3) When a stalemates sets in. In this case, if someone refuses to vote the draw within some turns, mods may step in and investigate positions. Sometimes they've forced the draw.
4) Also, since voting Draw IS a diplomatic tool (unlike Pause and Extend, see the Rules), you may want to vote it even if you actually don't want a draw. IE, in a gunboat, voting draw can help you find allies. So if someone is voting draw and it doesn't look logical to you, it may be:
a) he's trying to communicate something
b) he's underestimated his position
c) he doesn't know what he's doing :)
d) your logic is not so strong! :)))
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