Up to a point - yes.
There's a certain width, where text begins to become annoying to read if it's too wide. Easy to test if you have a large monitor, just grab notepad and paste some text into it without line breaks.
However, when going down to smaller sizes, being fluid might be a good idea. Mobile phone browsers are more and more capable of displaying "normal" websites just fine, but they are sometimes width-constrained, and as such, benefit if your site can fit in a bit smaller space.
Personally I also like to keep browser on my monitor but only at half of the monitor's width (24"). Sites which scale nicely into that are very good.
I think it's mostly a user convenience case. Not all sites will benefit from being fluid, but I think sites which have lots of text content are the ones that will most benefit from it, at least if they are fluid up to a max width (say 800px or whatever)
