I always thought performance based pay was a very right wing proposal, supported by right wing organisations.

I think that's a good point -- unions are not perfect and may support the interests of their members over the interests of other workers or society at large. However, in my view they are far preferable to any corporate board of directors, which represents only the interests of the owners of the company. Politics is struggle, it isn't just opting out of things you don't like, or else you'll end up a hermit. it's organizing for representation and power, and unions are one example of that kind of organization. An imperfect one, yes, but far preferable in my view to none at all