Usability testing can (and typically does) take place at any stage of the software development lifecycle, so beta testing is just a point in time when you decide to go and do the testing part of your overall UX design process. The key point is to try and collect information that will help you with future UX research and testing activities, taking into account the potential pitfalls of beta testing and trying to complement it with other forms of testing.
Personally I don't see UX and beta testing as different activities, it is more of a continuous process in which testing at a particular point of the software release is seen as doing beta testing. Of course, this is purely from a UX perspective as beta testing has other implications in terms of analyzing software performance, product development pipeline and marketing activities as well, so it is very useful to communicate with other teams to make sure you don't miss out on vital pieces of information that will help improve the overall user experience.