Oh, I have no problem at all with you playing Devil's Advocate. I actually kinda like it, because it keeps the discussion going and really makes me think about my points. And the more this debate continues, the more positive I am that I'd rather keep Cassell and Wally. Back to your points...
1) Kidd is not nearly the pick-and-roll guard that Cassell is. His mid-range game is incredibly inconsistent because his jump shot is incredibly inconsistent. Teams would not have to respect his shot nearly as much as they do Sam's which would mean fewer open shots for KG and everyone else out of this set. In fact, half court offense in general is an Achilles heel for Kidd. Its not a coincidence that everywhere he goes (NJ, Phoenix, Dallas) becomes a great fast break team that breaks down if the opponent refuses to let them run.
2) With this trade, our lineup becomes Kidd, Spree, Hassell, KG, and Kandi with Hudson, Hoiberg, Madsen, and Erv as the primary bench contributors. Outside of KG and POSSIBLY Hoiberg there is nobody in that top 9 with any kind of consistent offensive game. Kidd, Spree, and Hudson are streaky shooters that either score in buckets or not at all, and the other players are just not as offensively talented. With the offense that we run, I don't like that at all.
3) The break. I'm sure that with his rebounds and steals, as well as his general tendency to push the pace, Kidd could create more break opportunities. My point is that none of our centers are transition guys (Erv or Kandi run?

), and KG tends to be our last line of defense/best rebounding option so he very rarely runs himself. That leaves Kidd running with Hassell and Spree. Now don't get me wrong, that would be a great thing (I wish we did run more often), but I don't think a few extra fast break points from Spree and Hassell would make up for the diminishing we'd see in our half court offense.