I read fairly regularly, and hear more often, that "developers don't make good testers."
Of course, I also see, and hear, just a little more hushed so as not to offend delicate sensibilities I think, that "testers don't make good developers."
To both of these assertions I'd like to offer the same (heavily edited) response:
People learn skills. Very few are born innately knowing how to write code (in any language). Very few are innately knowing how to test software and report the results in a meaningful way.
These "testers" who go on about developers not making good testers have no idea what they are blathering about. Likewise, "developers" who like to talk down to testers, or talk poorly about testers when there aren't any around, are likely insecure ninnies.
Being a good developer means you also test. These days, being a good tester means you can at LEAST read through and understand code.
The hardest part of both jobs is to admit you don't know something. The second hardest part is to communicate clearly and openly.
All of you, get over yourselves and learn how to work together.