Sabre/Seraph are space fighters. Not designed to really be used outside of Long Night of Solace. Seraphs especially because they’re supposed to be AI only vehicles.
The Pelican shouldnt be flyable at all. Besides we literally have the Transport Falcon which does the Pelicans job unless you wanted an easy to control gunship. I do too its just Halo Reachs pelican sucks just as much.
Actually, both the Seraph and the Space Banshee have a prompt to use them, it is far from being AI-only.
It's really to make people happy but they aren't good for the current multiplayer, the Sabre and the Seraph aren't made to land (and flying vehicles much like the Banshee, nothing to impress me) and the Falcon is indeed a proper transport (properly scaled for multiplayer) even if Elites can't use passenger seats.
They could be used if the gravity barrier + the soft kill zone were pushed back (or removed) , I suppose a creative map maker can build something to land those things without dying.