Au contraire. GV's can arguably be the most effective (to say nothing of efficient) assult weapons depending on the circumstances.
If you have a v-base with a spawn to an isolated port, the GV's would be a better choice than dragging a CV over to the port and using aircraft.
1) transit time of the CV
2) vulnerability of the CV, and massive setback if the ship is sunk
3) digression if a port is flashing, but no darbar or radar blip is visible, few defenders will up.
4) equally effective if correctly used
Also, a single panzer out ahead of the troops can eliminate any GV's in town if he is sufficiently skilled. He can also do clean up on the town, letting the aircraft focus on maintaining a cap.
The use of GV's also allows the group of attackers to be smaller and still keep the base mostly intact. instead of two groups of planes, 1 to cap 1 to kill town, you only need 1 group of planes to cap, and a smaller group of GV's to kill the town. 80 HE shells will kill more buildings than 2 500lb bombs and a couple rockets would anyway.