First off I was talking about Skirmish matches, and I guess you are talking about Galactic Conquest. In Galactic Conquest, I wouldn't use the Neb B's either. I would have one or 2 in back up so they could take out tartarns, but I would try to get on without them. In skirmish, its worth it to take the Neb Bs to get rid of Tartans quickly and to draw out the enemy fleet. You want to do everything you can to minimize your enemy concentrating fire on your heavy duty ships - the mon cals. Sure, 2 Neb B's cost more than 1 Mon Cal, but with the enemy drawing fire away to get rid of the anoying frigates it frees up your mon cals. With the shield boost on the frigates (I know mon cals have it also, but not nearly as effective) they can take a long time to destroy. It takes longer to destroy 2 Neb Bs than 1 Mon Cal which means other units of your fleet can live or at least live longer because they aren't getting shot at as much.
Micro isn't as big a deal in this game, but I play a few games where it is absolutely everything. Empires: DMW, for example, and especially in Imp age, has a lot of micro. The whole idea of the micro is to get every unit to shoot one unit of its counter (difficult to do with 1000 troops on a side). Say person A has 100 infantry, but is just letting them fight without any direction. Person B has 100 identical infantry, but is microing. He is selecting all of his infantry and giving them one target at a time, eliminating it imediately. Then he goes on to the next and so on. He will end up winning with over 20 infantry left. (Trust me I've tried it when showing somebody in my clan this) and if the person is particulary quick you can have about 30 infantry left.
Sorry about the off topic bit but it is relevent. The point is, your army is more effective if your units aren't getting concentrated fire. Its better to have a bunch of injured units that can fight back than to have a few units of perfect health (the few units would have perfect health because they would either be dead or never attacked).
The Neb Bs cause the enemy to spread out fire more, which actually saves other mon cals from dying. Its especially more effective if you are in 2v2, 3v3 or 4v4 games where the battles are large and most people can't micro effectively.
*pant pant* Sorry about the long post and if it doesn't make sense just ask and I will explain directly...
"It seems to me the humour of Sails is so dry you could strike a match on it and it would be the humour that caught fire." - Friend of Old