It gets really messy with plugins, Java, etc. Browser developers have had to jump through all sorts of hoops to get 32-bit plugins (aargh, Flash!) working in 6

It gets really messy with plugins, Java, etc. Browser developers have had to jump through all sorts of hoops to get 32-bit plugins (aargh, Flash!) working in 64-bit browsers, (e.g. on x86_64 Linux distros) but it's presumably even harder to do the opposite, use 64-bit plugins from a 32-bit browser. My x86_64 Linux distro has 64-bit Java, for example, which won't work with 32-bit browsers. Aside from that, it won't be long until 64-bit operating systems take over even on the desktop, even in Microsoft-land, and the less 32-bit cruft that is dragged around, the better.