Windows 7 specs out at a *minimum* free drive space of 20GB for the 64 bit version and that would likely be very tight for a working Windows machine. Given the limited drive space on an SSD (especially if you have the 128GB rather than 256GB drive) I opt for virtualization largely so I can retain one single drive partition rather than having to partition between Windows and OSX. That is, you can have a small Windows virtual drive and then share folders from OSX that will allow using the space as needed by each operating system.
Of course that requires buying Parallels or VMWare Fusion in addition to Windows, but I figure it's more than than worth it just for the flexibility it gives.
That said, on this 13" Air with a 256GB drive I have an "inherited" 80GB Windows 7 virtual machine (came over from the old MacBook Pro). On my 11" Air with a 128GB drive I opted to install a new virtual machine on an external SSD, but I don't often need Windows on that machine.
The main downside to virtualization is a slight loss of speed, especially on high end video intensive applications (meaning things like gaming). However the Intel video chipset in the Air is hardly optimal for that sort of thing to begin with, so I wouldn't have gone for an Air for a machine that was going to be dedicated to doing a lot of that sort of thing.