Well, this is somewhat better at lower levels where spells which actually create physical barriers are hard to come by but any Image spell can, it seems to me, be used to replicate wall spells (or to create chasms or other obstacles) as a way to divide the opposition which will not also inconvenience the party (which knows the obstacle is an illusion).
For instance, the party encounters 10 Orcs in an in-doors enviroment, five of which are close and the other five of which are farther back. The party mage casts a Major Image of a wall rising out of the ground between the orcs so that the ones farther back are tricked into not being able to aid the ones in front. This is also good for trying to obstruct archers or to trap people by creating the image of a circular barrier around them.
Altin