IMO, It varies by market. AC's are not like CHR, classic rock, classic hits, Urban, etc. Depends on the competition. AC stations in markets where there is a fairly strong Hot AC/Adult CHR may play more currents. WLTW is liberal with currents as of late and they're competing with WWFS, a Hot AC. Remember, a LOT of Hot AC's have evolved out of the 80s, skipped a lot of the 90s, and are pretty much "Adult CHR" now and don't resemble AC stations like they used to.
I agree that normally, the AC does better than a Hot AC/Adult CHR/whatever. Maybe it's just a dry spell for the format, like I keep saying, there aren't many extremely good currents right now like there have been in the past few years. There is a decent amount of material for the H/AC format currently, which may be helping them.
I always come back to my argument that AC stations that play more older music typically do better. With the right songs, I think you can snatch the 35-54's and keep them happy. It has to be the right songs and the right balance of eras. For example, saying you play 13 songs an hour...I'd do 2 '70s, 4 '80s, 2 '90s, 4 '00s/recurrent, and 1 current. If there's extra time, throw in another 80s or maybe a "new" current in light rotation. 2000s and 80s are the most important decades. I just don't know about more than one current, then you may risk running the 40+ listeners off....and is there really enough material? The right 70s songs will definitely attract the 40+ audience and won't hurt the younger end. Same for the 80s. Listen to WSNY in Columbus; for an AC in 2011, I think they do it right. I think it just depends on the market.
In my opinion, when the 70s and especially the early-mid 80s get too old for AC, there WILL be a problem. The very late 80s, 90s and early 2000s aren't going to give the format the quality and abundance of material that the 70s and 80s have. I've often wondered if this will be the death of the format.