Namely, because they allow something like Adobe Flash Player - which doesn't come close to supporting all the platforms the Web runs on - to become a defacto standard. Thus restricting a large portion of the Web to just a few platforms Adobe wants to support.
That said, nothing about what I'm proposing specifically requires plugins. All it would require would be for the browser vendors to work together (cough, cough, I know, cough cough) to implement a standard mechanism for doing this. Actually, it might not even take that. Browsers already have a way to setup handlers for unsupported content types and what-not, so it might be possible to build what I'm thinking about largely on top of that. Of course it would mean that if you wanted to run an "application" UI you'd have to have a suitable platform (an X server, or something like an X server) running along-side your browser. So you maybe wouldn't be able to run OpenOffice on your smart-phone. OK, personally, I can live with that. Not all devices are equivalent and there's no reason to expect every thing to work one every device.
URLs are great, but they don't have to be limited to the web. Or, rather to say, the thing on the other end of the URL doesn't necessarily need to be something the browser handles directly.
I'd like to see something developed that lets you do something like:
where clicking on that link in a browser launches the remote app and then renders the UI locally using X11 remoting - as opposed to trying to render the application UI in the browser.OK, I know, go ahead and say it.. X11 sucks, X11 remoting doesn't work on WAN links, etc. To which I say:
a. Fine, let's invent something better, that still avoid the need to pack every ounce of functionality in the universe, into a web browser.
and
b. That doesn't jibe with my experience anyway. Just earlier this week I was playing around and decided to launch a remote X app using X forwarding over ssh, over a public Internet link. Worked like a champ. In fact, it reminded me of how fucking awesome X11 remoting really is, and makes me long for either a resurgence of interest in it, OR (see a above) the invention of a newer, better version that everybody can be happy with.
There's also a lot to be said for delivering applications using Java Web Start as well. JWS is wicked cool technology that is tragically under-utilized. IMO, anyway. :-)