Gee...do you think in retrospect Saban wishes he'd gambled on Drew Brees' shoulder?
The reason Saban went with Culpepper over Brees is Culpepper's ability to deliver the deep ball. Brees has looked good, but Saban wants an offense that can stretch the field. You wouldn't know it judging by the play calls this season, though--Miami is one of the least vertical teams in the NFL this year, compared to being second-most in 2005. A lot of that is on Mularkey, but also Harrington's inaccuracy throwing medium and long balls.
Don't be fooled by the Detroit game; it's Harrington's ONLY good game this season. Unless he plays that way in every single game from here until the end of the season, there's no way he's challenging Culpepper for starter status in 2007. Of course, if Culpepper can't return to form, then it's a different story...