[SixOfSpades]

The "no Dualing to a kit" rule is due to TSR's standards of "enough" specialization. A player building a Thief can build a backstory about how they have trained to use their skills to sneak up on and immobilize their captives (Bounty Hunter), or dispatch them instantly via application of poison or slitting their throat (Assassin), or engaging in straightforward, non-sneaky combat almost like a Warrior would (Swashbuckler). They can also decide that they want to learn how to wear armor and train in more weaponmastery (Dual to Fighter). But, apparently, it is not as believable that they would, say, suddenly develop a deep distrust and hatred of spellcasters (Dual to Wizard Slayer). According to TSR, an Assassin is a splinter group of Thief, and a Thief->Fighter is another splinter group of Thief, and that is all the specialization that needs to be done.

I disagree on this sense: The setup described is analagous to saying that "A brain sugeon can change careers and become an auto mechanic, but a general practitioner cannot become a German auto mechanic." Personally, I would allow a character to Dual to a kit as long as they are not Dualing from a kit. There are some class/kit combinations that should be restricted, though: Swashbuckler/Fighter being the most obvious contender, and the various Kensai combinations close behind. I don't mind the Wizard Slayer->Thief Dual, since the UAI is most often used to wear items that the Wizard Slayer should have been allowed to use in the first place.