I would also not make the trade, but it would be a tougher choice for me than it seems like it would be for you guys.
Pros: Garcia is as good or better than any wing backup we have, and at 6'7 he can play some 3 too.
Pro: I believe Casspi can be a very good player someday. A legit starter and possibly the 3rd or 2nd best player on a middling to elite but not quite contending team.
Pro: Jason Thompson is a viable backup 4 or 5 for us, that offers more size than Baby.
Con: Glen Davis and Kendrick Perkins both have shown a grittiness and toughness that Casspi, Garcai, and Thompson might lack
Con: Nobody in the trade can D up 5's like Perk. We'd lose our best post defender, on a team that needs that.
Con: Casspi and Garcia are quasi redundant.