- They're more secure - filevault, a linux kernel, and generally small virus surface area all help.
- They're way more stable - see above RE: linux kernel vs Windows.
- The vast majority of your work is probably done in a browser now, or at least should be, rendering desktop apps obsolete. Sure, things like Photoshop or Omnigraffle are still best running natively, but Omnigraffle (the best wireframe/flowcharting program out there) is only available on Mac.
- OSX is easier to use for common users than Windows - and that means lower support costs.
- They're now more manageable in the Enterprise.
If you're running a laptop without an encrypted disk, throw it out, encrypt it, or switch to a Chromebook (http://www.google.com/chromebook/) so no sensitive data is stored locally. As a business user, you can't afford to leave your laptop in a coffee shop or airplane with an unencrypted drive. It's just far too easy for blackhats to read your bits.
Now that Macs are easily and powerfully managed, and most software is in the cloud, there really are no more excuses.
Also check out Simian, which enables hosted software deployment for Macs. Open sourced at http://code.google.com/p/simian/