When Jimi Hendrix did it I think it was between him and the guitar. They had a "special relationship" that almost certainly included some hate. Pete Townsend was the certified bad boy of his time. The Who were always the worst. They had a reputation to keep up. The first time I saw someone smash up their guitar on stage was in 1980. It was Paul Stanley of Kiss. I even secured a part as he was tossing them into the audience. But it was of course not his good guitar. Everybody could see that.
My thoughts on this is that if they are your instruments, they are yours to do whatever you want with, including smashing them up. However, I fail to see what's "badass" about destroying your own property just for the hell of it. If it's a part of some greater show of message, sure, why not, but the general rule of thumb is that musicians should be careful with theatrical antics. It doesn't always play out as they had imagined it would.