- It sounds to me like you should just use display:none and write a quick function that toggles the display property for the right-hand menus/lists.
Due to their large filesize, I try to only use the Y! UI library if I need some widget it already implements, or if I'm trying to do something complex that might be susceptible to cross-browser incompatibilities (ie AJAX calls and DOM manipulations). To me, if I'm going to force someone to wait for all that code to download, I better be doing something with the code that will ultimately buy back that time, making the application easier to use.
Now, if you already have a need for the library elsewhere and it's just a matter of implementing it in another location, I'd say go for it--use the library.
Just my thoughts,