Allen was drafted for his versatility, and Saban hoped to turn him into a DB/CB hybrid, so Saban trying to teach him two positions at once obvioulsy hurt him. Putting him at CB was the next bad move since I could've told you from the get-go that he wasn't fit to be an every down corner.
He made some plays when he was switched this year, but people need to remember the only reason he was playing was because of injury, and a guy we signed off the street, Lance Schulters, came in and immediatley equaled and, in many respects, surpassed Allen's production.
Allen was caught out of position this year more times than not, and celebrating after making tackles 8 yards after the fact, I'm sure, didn't impress Parcells.
If he's to make it in this league then he needs to dedicate himself to getting into position on pass coverage. There are plenty of safeties that would come a lot cheaper than Allen that could easily do what he does.