Role of Tachos in EMC / Servo Systems w/ Encoders

My understanding is that EMC servo systems using motion controllers
like Servo-To-Go or Pico connect encoders to the motion controller
and tachometers, if present, are connected to the servo amps.

Since EMC is the brains of the operation, why isn't the tacho
feedback directed to EMC via the motion card rather than to the servo
amps--the brawn? What use can the servo amps make of the tacho
information? The plus side is that at least the servo amps I have are
all set up for a tacho voltage signal, so it's one less thing to be
interfaced to the motion control unit.

This question must have come up before, but I couldn't find a
definitive thread in the archives.

Thanks in advance.

Jack