I do believe it's overall a small number. You are correct the 61.5/110/119 setup was used before 129 was even thought about, by far most users of HD have subscribed in the last 2-3 years. In that case, the western arc or more recently sometime the eastern arc was used. Also, many early adopters of HD also frequently make changes to their setup. I have had three different setups to recieve HD since 2004. Dishs plan is obviously to have all customers on WA or EA. This most likely is to maximize their available bandwidth.
I don't think it is that simple. 129 was up there when I got HD, but I still got a wing dish. Northeast customers never got 129. I think when upgrading some existing customers, some installers prefered to add a wing dish to existing Dish 500's. For others, HD locals were on 61.5. And lastly, some who had internationals used 61.5 for them and HD. There were plenty of reason to keep doing mixed installs even after 129 came on line.