When you have dedicated testers in the team there is a risk that the developers have the attitude that they don't need to test their code thoroughly because someone else will test it for them.
Then the testers receive code with a bunch of obvious errors that could have been easily spotted by the devs if they just made some basic tests. This is obviously irritating. The developers get a bunch of bug-tickets and get defensive. Both are now irritated at each other, and the irritations grow stronger and stronger, every week.
That's a worse case scenario though :) It doesn't have to be like this. Most of the teams I have worked in, developers take full responsibility of testing and we have no dedicated test resources. In my experience this increases the quality and speed. And less irritations!