encoder installation on servo drives

I asked this on cnczone, but no replies. So...

I've noticed that almost all the servo-based setups I've come across:
1. Utilize a belt drive to connect the servo to the leadscrew.
2. Place the encoder on the servo.

I understand the reasoning for the belt drive, but wouldn't it be
better to put the encoder on the leadscrew shaft?

In other words, doesn't the belt drive add some backlash effects with
the encoder on the servo?

Or does the slop help prevent servo hunting when there's no motion?

Thanks.

tom