Most of the Primes ARE useful, the Osiris, Raven, Uziel, Warhammer, Mauler, Awesome all have great Primes.
But then you look at bad primes, Bushwacker, Thanatos, Atlas come to mind.
I would rate the Owens, Catapult, and Hollander in between because the Primes themselves are even specialized.
The more I play the more I realize I wouldnt be caught dead in any other vairant than the Prime on the Uziel, Warhammer, or Osiris.
And those three can only get better in the future, the Osiris could get more armor, and more lasers, the Uziel and Warhammer better PPC behavior and they could all benefit from DHS, in fact the Uziel and Osiris were built around them.
I'm frankly amazed at the Warhammer, that is some solid old skool tech right there, you could mount Dropship railguns on that thing and there would still be critspace and heat capacity left for a few MBL.

The poster child for bad jack of all trades prime performance is the Bushwacker, look how much it has been buffed up already and it still stinks. Then there is the Thanatos, its Prime is ok, but the others are better.
One thing I have found is that particularly with the Thanatos variants, they are good against certain weight classes, you should not think of a Heavy mech as automatic counter to another Heavy mech, and you should also not think of it as automatically good at killing lights either.
For instance, the Thanatos B, great at cutting down light mechs from afar but having the speed and JJs to dance with them if they get close, but the firepower is simply insufficient vs heavies and even mediums. Now take the Thanatos C completely opposite, great at running circles around heavies with MASC and has close rock em sock em weapons and killer flamers, but try catching a light mech with any of those and you will find it to be a nightmare.
Now take the Thanatos Prime, has the lasers of the B but less of them, and the harder to aim punch of the C in the MRM.
It is decent vs both, but great vs neither, while being quite good at combating enemy medium mechs, the problem is this game tends to go in waves, ALL LIGHTs... until... ALL HEAVIES.
And always always always remember, once we start getting 4 novacats and 2 madcats onto the field, say night night to balance, its officially a joke trying to balance jack of all trades workhorse mechs against a couple of those superboats, once the game gets to that stage, fun seems to drop off for me much of the time.
WE NEED MOAR AWESOMES AGAINST THEIR NOVACAT C'S MOARRRR ISAYYYYYYYY!!1!1111