When I bought/built my first server I went out and bought an internal USB card for this exact reason. Then the logical/Worst case scenario (read always negative) side of my brain came up with this...

I leave my dongle attached via a long USB extension secured to the case via screws...here is my logic...
If someone gets into the venue and sees 2 bright shiny MacPros in cases they are going to grab the machines and go, they aren't going to worry about the dongles/keyboards etc...I could walk into the venue and see that the servers are gone I wouldn't be happy about it but I could go out to my local apple store and drop my ccard and buy a new MacPro some hard drives use a backup content drive to transfer over what i need and be back up and running in a couple of hours...

If the dongle was an internal USB card, someone steals the MacPro, and has no idea as to the value of the dongle and tosses it in the trash...