No. You have to use a roster spot for Barkley. They could have just of easily put either LT or Sims in that roster spot if they were in fear of losing them to waivers. Either way a roster spot is taken by a 3rd string QB.
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.