I guess it depends on how you use your bookmarks, but personally, the nine speed dial gives you isn't even enough for the ones I keep in my bookmarks toolbar, let alone the whole bookmarks menu.
On this PC I've got Windows XP and Gentoo in a dual boot, and sometimes I pop into Windows 7 in VirtualBox on my Gentoo partition. If I bookmark a site in any one of those three OS's, it'll automatically show up in the other two the next time Xmarks syncs. And that's just this computer--same goes for the one I use at work.
For someone like you, who's constantly setting up new systems, I'd think having Xmarks would be even more useful. Every time you install firefox and xmarks, all of your bookmarks are there immediately, without transferring any files manually or trying to remember everything on your own.