some don't like it, but even with monitors of any size, there is not enough space in typical layouts, (i have three 24 inch monitors so I can go dual or triple wide) XSI, Maya and Houdini are much easier to use in dual IMO as you can drag the views/menus to the other screen and not be in layered window hell.
With XSIs new explorer object viewer dual setups will be even better as the limitaion of not being able to have more than one view manger to put on each screen will not be as much of an issue.
Dual is usually not much more or even cheaper than one big fancy monitor.