Yes, I think they are useful. This is one reason I'm not wild about Judith Butler or gender theory.
I think gender is a totally unhelpful and crappy concept that we should get rid of. I think talking about 'performing' gender attributes agency to people who do not have that agency.
I am aware Butler herself sees both similarities and differences between gender 'performance' and theatrical 'performance', but many people who use her work only focus on similarities. And this tends to lead to a really unpleasant situation in which 'gender performance' is seen as something comparable to theatrical entertainment, whereas for many women (and men, and perhaps especially homosexuals), it can be a matter of life and death and extremely stressful.
I think that claiming (eg) cross-dressing as an exciting subversive 'performance' of femininity, as Butler does, totally misses the point that much cross-dressing is pretty misogynistic. And it misses the point that these activities do not create a new and excitingly fluid concept of 'femininity', they further entrench the old, patriarchial, tradition ideas about what 'femininity' is.
I think it's a massive pile of toss.
There are some aspects of queer theory and of JB/JB's disciples' work that occasionally seem quite interesting and valid. None of them seem to me anything that we couldn't do from a radical feminist perspective, and do much better that way. So, for example, someone I know did a talk about how cultural assumptions about gender shape biological teaching in schools. Radical feminists know this is true. We've talked about the myth of the hymen on this forum loads of times. It'd be possible to discuss that either as a gender theorist ('ooh, look, it's all performativity cos the body isn't how biologists used to think it is!'), or it'd be possible to discuss that as a radical feminist ('look, the patriarchy lied about our bodies'). IMO the latter is simply more useful.