"Economically, you can think of a startup as a way to compress your whole working life into a few years. Instead of working at a low intensity for forty years, you work as hard as you possibly can for four. This pays especially well in technology, where you earn a premium for working fast."
http://www.paulgraham.com/wealth.html
Death march conditions are not sustainable forever, but I think most of us have had a year or two when we were insanely excited about some project and enjoyed working 60 hour weeks to get it done.
Paul Graham's words suggest the rational case for extremely long hours is when you think you might be able to relax afterwards, perhaps retire early due to the success that you are reaching for.