As someone who has used http://www.vanmiddlesworth.org/vitrite/ to toggle always-on-top and transparency for a long time, it is strange to see this -- browser-specific -- reinvention of basic window management.
On the other hand, my desktop is 2560 pixels wide, and as a result, I've never had the need nor desire to maximise anything. I also rarely use multiple tabs, preferring separate windows instead, which I can then arrange as I please. I wonder if the popularisation of tabbed browsing has decreased the desire/visibility of comparing/contrasting in general, since as you mentioned, it is much more difficult when you can only see one site at a time.