Perhaps for things like Google Docs or Campfire it could be useful? When the website is a bit more like a desktop application.

But for something like Fluther or just general web browsing, I don’t really see the point, personally.

I use fluid for running Highrise. Partly so I don’t accidently close it along with other windows in Safari (which I have a tendency of doing) and partly because I want the window to be a different size and setup (not showing the tab bar, address bar, etc) than Safari.

Like richardhenry said, it’s mainly for using a site more like a desktop app.

Fluid has completely changed the way I work and use a computer!
I’ve moved everything I do (email, calendar, chat, blogging, word processing) into the cloud: I do everything on the web. Trying to use applications within a web browser never quite worked for me. I’d end up with 5–10 open tabs and no convenient way to switch between them. And one browser crash and I lose everything.

With Fluid, you can command-tab between different programs (just like normal os x applications). Plus they run completely isolated from one another. I currently use 8 SSBs (site specific browsers) and almost all my work happens within a Fluid instance.