Serial consoles do not appear to have much going for them. They are slow, require special null-modem cables, and do not provide a graphical interface. But what is going for them is considerable. Serial cables are standard equipment and can run over RJ-45 cables, can run up to 200 ft (about 100m) at 9600bps. Serial concentrators can run the consoles of over 32 ports into a central box, so all the consoles in a cluster can be accessed from a single location.
You will not require a crash cart, KVM switch, or keyboard, montior, or mouse. Because of the serial concentrator, you can access the console of a machine in a colocation cage from your desktop.
The real limiting factor so far has been that even though you can access the Linux console via a serial port, most x86 hardware was not set to send its POST and BIOS information to the serial port. More and more "server" motherboards are starting to include full serial support in the BIOS, so you can access the BIOS and make boot changes via the serial port.