Well first of all, whether there was a significant benefit might be irrelevant, just like we don't hunt down all gnats in the world. (Of course if we could hunt down all mosquitoes we probably would but anywho...)
But he probably hoped that once he took over they'd come around and be useful in stifling the few remaining Toa and the like.
I doubt the Brotherhood considered the Hunters 'gnats'. The Shadowed One and many of his Hunters were fully capable of fighting a Makuta, and at one point had control of a Brotherhood installment containing Teridax's entire plan, that one of them discovered. Fortunately for the Brotherhood, it was Zaktan, and he wasn't about to tell anyone.
And your second point makes no sense. Once he was the universe, he used Rahkshi to drive them out of Odina. And why would he want to wipe out the Toa that remained, when it seemed he was having too much fun toying with them? He obviously didn't need or care for the Dark Hunters once he was in control of Mata Nui's body.
Don't forget that when he took over the universe, the DHs sided with the Order in opposing him. So of course he would then punish them. That changed the equation at that time. The question you asked isn't talking about that -- you asked why he let them stay for a thousand years prior to that decision. What I meant was that he realized that at the time that he would take over, that major change would force TSO to make a call -- abandon this silly war over such a trivial thing as the death of two incompetent members, or continue to oppose him.
Only at that time did TSO make that tougher choice, and that's when Terry punished the organization with serious force. It does make sense.
I figured you'd say he didn't need them too. Sure, but that isn't how Teridax thinks. He wants them, just like he wanted the Matoran. He wanted a lot of people actually on his side, because he wanted people's respect. That's how this all got started, after all. "I mean, really, who neeeeds any of this stuff?" (kudos for those who get the allusion ); he didn't really need the Matoran either. Could have just come up with Rahi to do the job and make Rahkshi do whatever they could handle, etc. But it's more efficient if you've got genuinely intelligent beings willingly serving you, rather than beasts and things better at destruction.
As for the Toa, he wasn't really in "have fun" mode. I'm sure after the serious matters of establishing his control with true security, ending the serious threats like Tahu and company, and then destroying Spherus Magna with Mata Nui on it, then he might want some to remain for fun (hence why I said "stifle", not "wipe out" ). But at the time it would have been better for him if the talented DH had turned their hostility away from him and toward the Toa.
As for gnats, a gnat will do some damage that to the gnat might seem impressive, but from the human's perspective really isn't. Nothing the DH did ever seriously threatened the Brotherhood. Spotting the Plan could have been an exception, but personally that seems unlikely. If Teridax was that worried about it he probably would have taken stronger measures to prevent anyone from seeing it, such as writing in difficult code, having special locks, etc. Had the DHs started using that knowledge to influence the Matoran and Toa, the Makuta likely would have turned serious force on them then and prevented it easily.
So more likely it's fortunate for the Dark Hunters that Zaktan kept his mouth shut.
Edited by bonesiii, Jun 04 2013 - 02:48 PM.