I don't understand your complaints about the classes. Here you see how often I use which class:
Most of the time I play Conquest and there are always situations where you need every of them. As good as the carbines are (I love my AK 5C), the assault rifles (especially ACE 23/L85A2/Famas) will destroy every other player every time. I use the AK 5C on engineer/support/recon and think of it as an avarage assault rifle. The real use for me is the RPG/Stinger/repair tool for the engineer, ammobox/C4 for support and spawn beacon/C4 for recon.
I think there are enough engineers in conquest, so most of the time I'm playing with Assault to wipe out infantry. But sometimes the own team sucks and they can't take out vehicles, so I do it on my own. I only play support/recon when I'm playing with friends in a good squad, they are extremly usefull for the squad and team. Engineers have unlimited rockets, you can provide the assaults with enough ammo and when I play recon I try to sneak to an objective, set the radio beacon there and wait for the squad to spawn there.
In terms of fun I would say: Assault > Engineer > Support > Recon. But when it comes to usefulness the assault and engineer change places.