I may have misunderstood the reason to try to be more selective in using the term [black widgets] as a search criteria.
I was assuming that if I were clever enough to figure out that some of my customers used the term 'large black widgets' to search then I could bid $.07 for the term [large black widgets] and get to the top of the heap for that specific keyword search.
NOT SO! I found that all my competitors who outbid me for the broad term widgets still came up higher than my ad even when the exact search string is typed.
For the widgets I sell, I actually find that [ThisWidget 2000] has a higher CTR and higher average position than the unbracketed search. While CTR and position can present a sort of chicken-and-egg connundrum, I think it was the CTR that inspired the higher placement.
So I would say yes, Irubin, there IS a potential advantage to bracketing your keywords -- higher CTRs and lower CPC. :)