That steel bikes have a noticable 'zing'/flex/compliance to them.
Bollocks, on road I can tell the difference, and the frames can still be stiff enough, off road frames just fit into varying levels of noodlyness, with 3lb steel frames at one end, and cannondale hardtails at the other.
Reasons to buy steel:
Cheep (if its not, seriously question your reasoning for buying it)
If it survives the impact once it will do it indefinately (so its good for my trail bike which also takes trips to the BMX track and dirt jumps)
I think they look better
Then again, a well designed 6.something lb aluminium frame would probably be just as good.