Useless in what context?
Isolation exercises have significant benefits for muscular hypertrophy, rehab, and in addressing some forms of muscular imbalance.
In the context of MMA the usefulness of isolation exercises is going to be a highly personal question. Unless you are rehabing an injury I suspect the primary benefit to isolation training is that you might look better than your opponent when your gi top comes off half way through the match.
I, for one, have never missed a rear naked choke because I could not complete a biceps curl. I've missed them because I couldn't keep my opponent stretched out and I've missed because my opponent was able to push to one side and drive his shoulders to the mat. Both of those are multi-muscle failures: My pull chain not being trained enough to maintain heal to hand tension long enough for me to finished the choke and leg, my core/pectoral girdle tension not being strong enough to fight his drive to the side and back.
The way my body fails when I roll leads me to train a lot of compound movements that require me to connect my leg strength with my pectoral girdle through my core. But that's me trying to address the points of failure that I see when I roll.
So you may need bicep curls, but i suspect that most people training in mma do not. Certainly no one training mma needs an arm day or a shoulder day. Okay, actually, 95% of the population would be better off skipping arm and shoulder day and doing more squats and power cleans. Most people need more systemic strength and will benefit more from the the system wide response that complex exercises provoke (increase heart rate, O2 consumption, neuro-muscular linkage, etc) than they will from a good bicep pump.
Unfortunately most of the population thinks strength training is body building. To those of us who don't think this way, the whole "Isolation Isolation. Isolation. 3 sets or 10. Should I have a separate day for arms or should I train them on leg day? By the way has anyone seen the foam pad for the squat bar so my neck won't hurt? Can I get a spot dude?" tends to makes a bit crazy and we get all grumpy and rude when the subject of bicep curls comes up. Hence, even thought you did not ask if you should do preacher curls on the machine before or after you do dumbell curls I've gone of on you like you have... I feel like such an a-hole for that - really.
Anyway, do your isolation stuff if it helps you, but for most people I suspect the points of failure in their games that could be addressed by strength training are not going to be helped by isolation training.