It seemed strange at the time, particularly since they had just taken a FB in the draft, but it's pretty clear that they took advantage of the chance to upgrade at very little expense.
Here are some interesting snap counts from PFF:
Javier Arenas (listed as a S even though he's a backup CB on Arizona's depth chart) has taken only 62 snaps for the Cardinals.
Quinten Demps and Husain Abdullah, by comparison have taken 256 and 102 snaps respectively.
Another former Chief, Glenn Dorsey, has only taken 9 snaps for San Francisco.
I don't know if either Arenas or Dorsey have had injuries this year, but they certainly haven't contributed much to their new teams.