[thejeff]

Depending on the paths: research, site searching, low level forging.
Mostly research, but anything they can do, so you don't need a higher level mage.
It's generally more cost-effective to have many cheap mages research than fewer expensive ones, even without research boosters.
How useful they are depends on the path.
D & S can remote site search. Sometimes you'll have boosters not in use that turn that can boost other paths up to 2 for site searching.
A lot of forging can be done with only 1 path. Lanterns, a lot of basic thug gear.

I usually keep low level mages off the battlefield, with some exceptions.