What is the purpose of taking level 10 as a thief?

Hi,

I've read Powergamming TNO thread  and I'd like to know about advantages of taking level 10 as a thief?

I am going to be a mage and I use all latest mods from this side (that changes some of gaining HP rules: Consistent HP: As per the game manual, TNO now always gains hit points as a fighter (1-10hp per level till level 9, 3hp per level thereafter) when leveling his highest level class, and no longer gains 1hp when leveling any of his other classes.)

Thanks.