Using Glass Scales instead of Rotary Encoders?

Hi Guys

I am using Gecko controls and Servo Motors on my CNC Mill conversion and
was wondering if you could use Glass type Scales (like the ones used for
DRO's) instead of rotary type encoders like the US Digital type? I know
allot of the high end CNC Mills use these glass scales for the feedback
to the controllers. I wasn't sure if the Gecko units would work with
the Glass Scales? I believe the glass scales produce a similar signal
to the rotary encoders? I think this would give you a really tight set
up, especially if the Ball Screws were direct drive to the servos.
Interested in your opinion both good and bad. Take care.

Regards
Bill