This trade may seem really weird since we just traded with houston, but I still think it makes sense for both teams.
Please note that I do not want to see Calderon traded, but it appears more and more likely that he will be.
Toronto: We swap an expiring contract for another expiring contract. We give up Calderon but we receive a veteran SG. Imo, Martin is exactly the player I expect Ross to become. Having him as a mentor for one year (at least potential to resign him) for both Ross and DD (and to a lesser extent Fields) would be very beneficial.
Also, it may help Lowry transition better with a former teamate.
Houston: They need a Back-up point guard, and they need more veteran mentors, especially for unproven players, *Cough* Lin *Cough*.
There are no long term penalties for either teams as both are expiring contracts. We are essentially swapping veterans to a team that needs their experience more.