Mobile Artillery useless?

Snipers are direct-fire, and should have vastly less range than artillery. They can easily coexist. The sniper unit would simply be the longest-ranged direct-fire unit. Snipers are the longest range role that can select a single enemy target and reliably destroy it.

Actually an advanced bot sniper unit could very easily double as an excellent scout unit, and would be extremely useful for gaining vision in the field in addition to its weapon.

Or dumbfire rockets/20mm unit for anti-land and anti-air?

Yes, but at the moment, mobile artillery doesn't have vastly more range than direct fire units. The artillery is fulfilling the role I imagine for sniper/rocket units at the moment i.e. single target, high damage units with range greater than a turret but significantly less than static artillery.
I'm wondering if this will be changed and artillery turned into less accurate, higher splash long range options with another unit filling their current role.