so much bike shop hate. there could be a few reasons that a rear derailleur would help in shifting performance. most notably, the springs will be stronger on the da rd. why is that important? isn't ultegra just fine?

i'll give you an example of a bike that refuses to shift well out of the box with ultegra: a specialized venge. i'm not sure how the cabling is setup with an addict, but the ultegra rd springs are not strong enough to get perfectly smooth shifting. don't take for granted that everything will just work perfectly out of the box with whatever parts bike companies choose to put on them.

specialized is aware of the issue and refers people to buy alternative oem parts from shimano. since every good bike mechanic will notice this problem, all of the springs are currently sold out.

again, while the op didn't say exactly why the rd was recommended, we don't currently know. but don't just think that a bike shop is trying to gauge you. if you actually have a mechanic who knows wtf is going on, they will be a giant help.