Heh. Once I decided I desperately needed Leia and Arissa, this seemed the natural place to go. Also, the only explanation for Jedi of any kind surviving the Purges -- like, ANY AT ALL -- is that there might be ex-Jedi à la Dooku who either kept their heads down or worked for the Empire or whatnot. In any case, Anakin trying to convince pretty much everyone he cared about to stick with the Empire made it seem inevitable that he'd do it with alt!Arissa, too -- and his expectation of success makes even more sense if he did succeed with someone.

(Though, admittedly, there are some ... complicating factors there, like he hadn't made his decision when he talked to Arissa and so it was less "join the Empire!" than "um, something weird is going on, something's going to go horribly wrong, you're not happy with things anyway, maybe you should get the hell out of Dodge." But I also think Arissa was on much more consistently okay terms with Vader than anyone knows about.)