The Blazers aren't a great free agent destination. They need to do everything in their power to make the city an attractive place for free agents. You can do that by moving the team to Hawaii and using the city as a lure, having excess billions to throw at the team, or make the environment one which fosters loyalty and respect for the players and staff.
I would tend to think that keeping Roy, and doing so very publicly, would be a very visible way to say, "We take care of our own in Portland." I don't believe the Blazers have done a good job of that (in the least), and this would be the first best way to signal a new era in which the organization shows more respect for its staff and players than most teams.
If this new approach landed even one free agent you wouldn't otherwise get, wouldn't that be worth keeping Roy?
It would take this and other steps to foster this kind of environment, but even if it's hard to quantify, I think the effect of keeping Roy and announcing a shift in strategy could ultimately good for the Blazers' chance of being competitive, and their bottom line.
I'm not saying this argument is the most sound argument you could make for either keeping or dropping Roy, but I hadn't heard it articulated, and I think it should be at least considered.