Here's the issue: The Cards want 2 QB's on the 53 man roster and 1 in reserve on the PS. How do they go about this in a responsible way?
1. They buy insurance in trading for Barkley. This insurance does not payout unless he's on the roster for 6 games.
2. Now, with this insurance you can risk Sims and Thomas on waivers with a reasonable expectation that one or both gets through, particularly given that most teams want to carry 2 QB's on their 53, not 3.
3. You sign one or the other to the PS.
4. You dump Barkley before the 6th game.
5. You now have what you intended... 2 QB's on your 53... and one in reserve on your PS.
You only way that they end up with 3 QB's on your 53 man roster is if they lose both Sims and Thomas to waivers. I'd argue that this is unlikely.
Barkley, for the first 2 or 3 games, is actually filing the spot open by Massie's suspension. When he comes, someone has to go. I think it will be Barkley, if we have a QB on the PS.
So... in the end the trade gave you insurance and cost nothing.