That's a tough call. Most traditional testers have little or no background in usability testing. It's fundamentally different to set up and test user responses than it is to test software for defects. User experience (UX) specialists are well-equipped to better understand how different design considerations are interpreted by users. Usability testing generally takes place early in a project cycle, and the end result is often a prototype that presents the look and feel of the application, along with aspects of its interaction with users.
That said, it's still a good idea to involve testers in UX testing. Testers have a better practical understanding of the end user, and in many cases have made significant contributions to requirements and user stories. I've seen circumstances where UX professionals have interpreted results in one way, only to have testers explain that the user community for this particular application didn't think that way.
How about seeking a détente with your UX experts? They have skills that will help you become a better tester, and you can be involved and helpful very early in the project lifecycle. I suggest that you propose a joint effort where traditional testers can be observers and consultants in UX testing. Make sure you know that they are in charge of the process, and that you are there to learn and be helpful.
In time, you may be in a place to do your own UX testing. If so, you will have the invaluable experience from those who have been trained to do it. Until then, you are in learning mode, and whatever you learn about usability will likely make you a better day-to-day application tester.
Finally, don't separate usability from software testing completely. If you find a serious usability issue while you are executing your tests, don't hesitate to open a ticket as you would for any other software bug.
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