Tom, you can take the suit off, it is too confining and kind of hot.

Choosing a crystal is a design consideration, and not a matter of "right" and "wrong."
Yes, of course, it is a design consideration, but way too often the 12 Mhz is chosen becuase someone read the uC data sheet without bothering to read the part about the UART. The design consideration should take all factors into account. If there is a valid reason other than "who cares, it works" for using 12 MHz I would like to know it.

Sometimes a project spec says that the board has to last nn hours on a 9V battery. So reducing the clock speed drops the power requirements resulting in a satisfied customer. Some of the designs I have worked on don't even have (or don't use) a UART.

Low clock speed, you can still use a freq that divides to a baud rate.
No UART, of course, you will select the xtal without concern for baud rate.

Have fun,

Erik