I did base my menus originally off the CSSplay site, but they are somewhat buggy. I remember tearing out a lot of hair around release 2.6 or so due to that. Eventually I followed a much more rigorous menu implementation based on Ruthsarian Layouts
, which really covered all use cases much better. I still have some elements of the original left in, but I got rid of a lot of its concepts.
I distinctly remember putting in an effort to support a fluid width drop-down but I did face difficulties (can't recall what). The fact is, the theme remains a hobby for me, and I don't feel inclined to go out of the way to add a new feature unless I feel I can do it quickly and/or satisfactorily. Menus are extremely tough to debug; in fact, of all pieces on the site, they are the toughest to get right. As I mentioned, I can look into this, but I have no plans to do so for the next few months at least.
Regarding the CSS vs JS question, the JS feature offers the capability to delay the drop-down and to fade it in, somewhat like a "hover intent" functionality. It was added based on requests from some users. Originally the menus were CSS-only and used to work across all browsers, but with the JS version in place I was compelled to break some functionality for the CSS-only menus in IE6.