Testers ask questions. Annoying questions. Where are the release notes? Does that change affect this other thing? If the app can do this, why can't it do that? Which third-party apps do we need to be compatible with? Where did this half-baked, unplanned, crazy new feature come from?
Testers check the quality of things besides the software you told them to work on. They clog the network stress-testing an app. They accidentally delete your source control repository if the permissions aren't set right. They critique the bug-tracking database. They suggest changes to the lunch-ordering process.
Testers delay releases. They always seem to find a serious bug when you're just about to push a new version. And now you're elbow-deep in backtraces, rather than basking in the warm fuzzy feeling of a release.
Testers cost money. Besides their paycheck, they want a Mac to test the app. And maybe a second one, to use while they're re-installing the OS on the first one. And then a Wacom tablet, because it sends different events than a mouse.
Testers are trouble. You don't want to hire a tester.