About a dozen times each month.
I understand that an internal unit might have been more convenient for you, but Apple tends to optimize for the more common needs.
The external third unit is pretty noisy, has a large wall wart, and takes up desk space which contributes two cables, power and USB, as well as taking up one of the small number of USB ports on the Mac. This is an exceptionally poor design compared to the experience of using a PC which reasonable comes with an optical drive, something that I use very frequently, as do most computer users.
It looks to me like they've tried hard to make sure that the included GPUs are adequate for 90% of their users, at least for today. As for tomorrow their philosophy has always preferred replacing the machine rather than upgrading it piece by piece.