If they do a better job ad showing ads you want to see than the official Twitter ads, then it adds value. But now there's no chance for competition -- you eat your twitter ads because you don't have a choice (aside from the obvious one of not using twitter, which I find wholesome and life-enriching so far).
I don't really feel more ads are valuable, no matter the quality.
Well they just have to be better than promoted Tweets I guess, then they would add value over the advertising your going to see anyway.