It's another riser for software-driven add-ins like modems or sound. Very similar to AMR or CNR.
The problem with ACR is that it's a VIA proprietary standard. That is, it's VIAs, and you'll never see it on anything but VIA boards, where as Intel has made its CNR standard available to everyone (including VIA).
Because of that, I think that ACR will go the way of Betamax. That is, nowhere. Those slots aren't popular as it is, and when they are used, people will be far more likely to use CNR (because it's an open standard) than ACR.