Another reason for using two or more CM15A's (and other computers) is the expansion of the number of flags available. I don't remember anyone mentioning this before.
Granted, using macros on multiple machines in the house make for a VERY complicated setup, but sometimes 16 just isn't enough. I currently have a CM15A set up with only the flags and macros for my HVAC setup.
Yes, I do have 3 CM15A's at my house and they get along fine with each other. My oldest CM15a has a blown out PLC transmitter in it, but it still sends out RF and receives everything just fine. I use it to send RF to another cm15a which translates that as PLC.
The only negative effect on this system is when I try to dim lights via my 6 in 1 remote. I get multiple dim commands.