I'd be very surprised if they traded Banks during this season. With Nash's health iffy, that leaves only Barbosa and Raja as guards. Banks can play defense, which is why he gets the nod over the vastly superior offensive player in Rose.
Next summer the Suns can shop for a push PG in the draft or through free agency, but right now their options are limited.
If they trade Banks now they have to get a P.G. back or sign a free agent that can at least play a few minutes.
But, like D'Antoni just said, when Nash is out we have Barbosa, and Diaw can play point guard!
I do not think they want to go that way for the rest of this year, but maybe they think Rose can play a few minutes.