Are two monitors really better than one?

I was planning on buying two new monitors and start using a dual monitor system for photo editing until someone suggested I go with one large 30" monitor instead. One good quality larger monitor would cost less than 2 good quality smaller monitors and and I'd save a bit of money by not needing a video card with dual output. Plus my desktop workspace would be less crowded to. So why do many photographers use two monitors and what are the advantages of using a dual system instead of one?