Yes, we have used the 'kids free' for Imperial Cities. I think if you look carefully at the promotions, the dollar amount always is about the same. ie. kids free, Disney Visa discount, early booking discount. They don't seem to vary much in the total amounts - just the promotion changes.
Actually, in thinking about it, I think the Visa disc was about taking a longer time to pay for it. My ancient memory isn't so great. The only thing I compared were the early booking and the kids free. Wasn't really a whole lot of difference between the two. Plus I think we got $100 each for being previous ABD folks. I guess if someone kept tabs on the various promotions they could give you far more exact figures than I can. I just remember thinking that they were very similar in $$$ for the three of us. Certainly not enuf to change my mind either way.
Might want to look at Costco, their travel section: if you book thru them you can regularly get $500 in Costco cash cards and til mid July (I think) it's up to $750 (total NOT per person) If you don't have kids and do have a Costco membership - it may be worth looking into.