You can connect an external display to your laptop. That's what I'm doing right now. Similarly for 2 through 6.
> 7. The desktop has lots of USB ports and they're all in use.
The external monitor will give you more.
I think reasons 8, 9, and 10 are all legit. I don't think there are any intrinsic benefits to going one way or the other. It all boils down whether you feel the time you spend building your rig is worth the money you save.