I dualed mine over at 7th level (th estart of the game). This way I could revive my fighter skilla shortly after Jon's dungeon. Also, felt that the xp expenditure (read time) was not worth the small gains going to 9th or (gosh) 13th level. At higher level, you will better options than going toe to toe as a tank. I do occasionally send mine into the fray, but only as backup. He is a mage first and foremost, although protection from normal weapons, haste, blur and a zillion ***s in two handed sword (wielding the sword of chaos) do pretty much make him a butt kicker. Hmmmm... Maybe I should have waited before dualing.
IMHO- the Kensai/mage is best for role playing. Your character could be the really intense type that discovers something new, goes all out to master it and then does something new. I think it would actually fit the type of personality that would become a high level magic user.
It has real possibilities as the main baddy NPC. Your party could be in conflict with an archmage that they have not met yet. He or she is aware of the party and wants to "know the enemy". So they meet and even adventure with a brash sword saint for a period of time, never being aware that their nemesis is amongst them.
We can even take a page from BGI and spell the name of the kensai "companion" as a reverse form of the archmage.
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- Cayle