What were your other QB options?
Cam Newton?
Unsigned guy
Practice squad from another team?
If Rivers is the only option or "best" option you might as well run a practice squad guy out there.
As stated before, Colts error was not looking to resolve this when AR got injured.
AR injury October 12.
Jones injury was around November 23.
Rivers started December 14.
So there's about a two month gap between Indy not having another viable QB on the roster, and being forced to sign Rivers.
Signing Rivers signals "our season isn't over, we're fighting to the end" but that shyt is over, Ballard/Steichen trying to protect their jobs by looking "competitive" over the last month.