Although Provoke could likely use a Stamina reduction (Same for Taunts and War Drum/ Warmonger) but I've never had a problem holding hate when I actually wanted to. If your GLA isn't Provoking becuase he doesn't have the Stamina, then he is doing something wrong, and should switch to Lancer for everyone's benefit, as his main concern seems to be damage.

There are many ways to keep hate, even just spam buffing yourself can keep hate on you, and the use of specific skills. Really, it is quite easy to keep hate, although, again, I wouldn't be against a Stamina reduction to Provoke/Taunt.