a thief mage has the advantage where traps abound.. he can still use a short bow/crossbow/longsword and well, its really only a problem if you want to wear armour.(but its no problem, usually you are wearing a robe anyway)backstab is invaluable, and when you start using invis spells to backstab you will agree.
no real reason why a f/m is better, unless you add the /t. (f/m/t)
remember, just an opinion..
hope it helps you out.
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