Never see one in battle field, there must be some sort of role for them right?

You see some mages during the beginning sequence in Helgen, shooting fireballs at the dragon, so mages serve the same purpose as artillery.

For some other purposes try:

http://www.uesp.net/wiki/Oblivion:The_Art_of_War_Magic
http://www.imperial-library.info/content/firsthold-revolt
http://www.imperial-library.info/content/capn-dugals-journal (part three)
http://www.imperial-library.info/content/2920-last-year-first-era (book six, chapter three)

battlemages are largely used in Wars in Tamriel, sadly the game doesn't show this as much as the lore :/

The Imperials had Imperial legion Battlemages in Oblivion where they were mages in heavy armor and in Helgen they were the same but in light armor.

They wear armor but use destruction and restoration magic over weapons.