without the brackets, your ad comes up for:
widgets make me feel black
black background with widgets
where can i find black widgets?
and so on, so if you do not care for any reason your ad may com up, then there is no reason to use brackets.
Put both widgets and black widgets in an adgroup and set a max CPC for the adgroup. Run the campaign for 24 hours.
Surprise! Surprise! You will notice that 'black widgets' will have a better CTR and lower avg CPC than just 'widgets'.
Always include as many specific keywords as possible. Your overall CTR will further improve if your ad copy has the keyword 'black widgets'.
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. :)