Eschatonic - 11:18 am on Apr 19, 2011 (gmt 0)
Presumably you're already matching to [buy red widget cheap]?
There are a couple of bonuses to longer phrase matches - the most important is that they are inherently more relevant which means a higher QS. Also, it allows you to track search traffic more accurately so you can identify further improvements to make.
The minor drawback to using longer phrase matches is that they're more long-tail, and will receive lower volume. But that's not usually a drawback as long as you have at least some volume.