Tachs and encoders

I am a little confused about using tachs and encoders. I thought I
had everything straight in my head about a servo system untill today.
I saw an advertisement by a company selling servo systems and they
used encoders ONLY, with no tachs. I thought you had to have both. Is
a system with the tach and encoder a better way to go? Are there any
disadvantages to using an encoder only? Any input would be greatly
appriciated.

Tony