You don't have to get both the Captain and the Lieutenant you only need 1 to get to the Major; I usually only get 2 of the 3. The Captain's Bazookas are often life saving for the USF and he has a thompson, but like the paratroopers and many USF units they take a lot of damage, and the Med truck is optional.
I concede that among these units you don't have to get all of them, But A rear echlon unit with 2 bazookas or a Rifle with bars won't cost you nearly as much to reinforce as the 50 MP model in Lieutenant and captain squads with less upkeep while performing the same function.
However my point is that the USF Has alot of units you can't really avoid that end up taking up alot of your popcap that might otherwise be occupied by more desirable units.