I have a personal story related to 12MHz crystal and baudates. My start with '51 was a "network" (RS485) of AT89C2051-based devices. Actually, I wrote the "master" software on PC, while we outsourced the "slaves". The 12MHz crystal restricted the maximum baudrate to 4800 baud. The overal response of the "network" was unpleasantly slow, and although I tried to cut down the number of bytes going through the "network" by sofrware tricks, the client permanently complained that the response is slow.
It would be easy to go up fourfold with the baudrate - there was relatively little data processing involved on both sides - but it was impossible to change crystals on the already installed units.
I was young and stupid then (...and I am not young anymore, so today I have no such good excuse for my stupidity...) and made more design errors, but this one stuck deeply in my mind and ever since I avoid the "decimally round" crystals wherever possible.

Jan Waclawek