The link title attribute appears in most browsers as a pop-up tooltip when the user mouses over the link. Given how little visibility that provides, I would think that Google would be insane to give much (if any at all) ranking weight to terms contained in it.

I know for a fact (from a Googler at Pubcon) that Google ignored the title attribute completely for many years. Not enough people used it for them to make it part of the algorithm. If they've added it in somehow more recently, then you might want to test for it. However, it's got to ne a minor signal, if it's anything at all.