Let me ask you something. Is there any reason to use Assault to get kills when Engineer can do it better?
So if you want to get kills on a more or less individual scale (be it vehicle or infantry) while not being able to help your allied infantry, you would be engineer.
Hell, it has RPG's that can destroy vehicles. But guess what? Support can be better too by mowing down massive crowds with an LMG.
So if you want to handle crowd control instead of vehicle kills, be support.
Recon cannot do
any of these better. They have no healing capabilities, they have no resupply abilities, no indirect fire weapons, no anti vehicle weapons, and no crowd control weapons. The only remotely helpful tools they have are the SOFLAM and MAV and those encourage the cowardly bush-person playstyle that make recons such annoying teammates.
edit as well: okay okay it is all clear now, we are talking about conquest, our mistakes
And offensive rush. Recons on offense in rush are absolutely worthless unless using a PDW and spawn beacon/tugs.
Defense they become more useful as an area-denial class but I'd still rather have another healer or AT on the field.