I too use arc...implying that people don't use arc is silly.
Yeah. People forget that DisplayPort was designed from the get-go by VESA for the computer market. I use it at home and love it. I run triple 24" LCD's on my desk in an Eyefinity setup using DisplayPort. That's its niche and it works well.
Home theater is a different animal and it takes a lot longer for standards to change / be adopted. All Joe Consumer cares about is that he can buy any device with HDMI and plug them up together. ARC is pretty useful for what it does. I rarely use it, but when I do, I think it's pretty nifty. I also know someone using an HDMI dongle on both ends to pass the IR signals from his remote into his media closet. It's a $30 solution that also works well in taking advantage of one of HDMI's many consumer-friendly features (the ethernet channel).