Fair but I think it's arguable that he's not very good based simply on his time at Purdue - he didn't have a ton of help/talent around him and the coaching staff there doesn't inspire much confidence in their decision-making (i.e. benching him after Marshall).
I think Mac saw how the lack of QB depth this year really tied his hands and he's determined to make sure next year is a stop-gap until we can get to the Franks/Allen years (fingers crossed). The more competition the better and, even if Appleby is just Del Rio's backup all year, he's likely a better option than Harris (*gulp*), Trask (who I completely agree is a ridiculous stretch), or burning Franks's redshirt.
Further, if he ends up being a viable option and contributes to us being successful next year, that 2nd year of success (after a 10-win Sec East championship season) helps raise the visibility of the program and hopefully inspires some kids to sign on in the next class who would've been on the fence otherwise.
Just my take. I think he's totally worth providing the opportunity for a year. The more options at QB next year, the better.