Frigates are awesome against things like special forces guard posts, but also they are the counter to about 1/3 of the units in the game. You will need them, and they have long been the favorite unit of many players, but depending on a specific campaign they may be more or less useful. If none of the AI's ship types happen to be in the 1/3 of ships that are weak to frigates, then you may not have much use for them. And, Pierre says, your play style could let you use some units instead of others, etc.
It's funny, with the ship types: someone has a favorite type, and someone else is convinced that same type is utterly worthless. Same is true of microfighters, for example. I think that boils down to differing playstyles and scenarios.