It's not arguable at all. They needed a second score to secure the win, remember the first play when the Steelers played the Colts in week 12, do you feel secure with a 6 point lead and 1 minute on the clock knowing the Colts can get a TD in 1 play?! It was the first time in two years Bettis lost a fumble, every possible stat leading up to that play indicated a TD would secure the victory and Bettis was the safest surest way to get the TD. That was the right call, in the playoffs you can't wait for victory to be handed to you, you must go get the win, that's what Cowher called and it was the right call, the kind of call SB winning coaches make. Kneeling and praying for time to run out knowing the other team can stop the clock 3 times is the kind of call perenial also rans make.
Sometimes bad things happen even when you make the right call, that's how sports work. A bad thing happened on that play, but it didn't make it the wrong call.
Proper strategy or not Cowher would have to sneak back to in P-Burg and clean out his house if that play loses the game.