Personally I find it actually quite desirable to run windows in parallers / vmware fusion and have ones professional productivity apps under it.
This allows you to easily backup your whole virtual working environment and even move it from a mac to another. You also separate your work from your personal internet browsing etc. which is good for security. Same is of course possible with Windows laptops as well.
For CPU dependant applications the drawbacks in performance are negligible and thanks to SSD's in modern Macbook pro's, disk performance is stellar regardless.
Real drawbacks are imo following:
-Battery life suffers when you have the virtual machine running (Like from 8 hours of web browsing to 6 hours or so (admittedly I don't have experience if this is the same with newest Broadwell rMBP's)
-Added cost of Windows & hypervisor licensing
-GPU compatibility & performance, though this varies from application to application. I have no experience with GPU accelerated 3D modeling or video applications under virtualization.
Benefits include
-Added security and easy backups of your work environment (ie. you stick a usb stick to your mac, it gets mounted in OSX instead of being mounted in your work Windows etc.)
-snapshots (ie. take a snapshot of your work environment, run windows updates / software updates without worrying, test for a day or two with easy way of going back if something doesn't work
-transferable working environment, just export and import your work-windows VM if you want to change your physical computer and you don't want to reinstall change anything.
-if you upgrade from dualcore 13" retina macbook pro to octa-core mac pro workstation with 64GB of RAM at some later point, you can move your virtual machine and increase the HW resources available to it with just few mouse clicks instead of reinstalling your operating systems and whole machine.
But like I said, all of these benefits are available also with Windows based desktop virtualizations. Mac + VM is good if you want to have OSX for something (personal use?) or prefer the HW / aluminum casing of Macs or something like that.