I suspect that 'emotion' and 'logic' don't separate as simply as we tend to think -- that they aren't separable 'down to the bone,' sort of.
I found this interesting:
Neuroscientist Antonio Damasio studied people who had received brain injuries that had had one specific effect: to damage that part of the brain where emotions are generated. In all other respects they seemed normal - they just lost the ability to feel emotions.
The interesting thing he found was that their ability to make decisions was seriously impaired. They could logically describe what they should be doing, in practice they found it very difficult to make decisions about where to live, what to eat, etc.
In particular, many decisions have pros and cons on both sides. Shall I have the fish or the beef? With no rational way to decide, they were unable to make the decision.
As far as depression, I find there are different levels or states. At a certain point, it does seem to offer a kind of icy clarity, but below that I start to feel like the brain-damaged people that researcher talks about. It becomes impossible to decide anything. I've actually at times made complicated charts and graphs when in that state, but it never seems to help. And then, when out of that state, it's simple again.
I guess the thing is that too much emotion can lead to bad decisions, so we think less is better, but apparently that also causes problems.
A video of the guy above talking about patients who can't decide due to emotional loss due to brain damage:
[youtube]http://www.youtube.com/watch?v=1wup_K2WN0I&feature=player_embedded[/youtube]
Anyway my $0.02 on the subject.