Spehro Pefhany fired this volley in
:

Is the third axis worthwhile-- given extra cost and additional hassles
to install?

It's a BIG hassle to install. For 99% of everything I do, I use only the
two axes, X and Z. It's fairly easy to do the trig to figure out what
moving the compound will do to the other two positions, if you really
need to figure it.

My DRO will 'compound' the compound and Z axis, but won't figure the
numbers for X at the same time. To do so would require an encoder on the
compound swivel and some pretty sophisticated algorithms I don't think
the average DRO possesses.

So it's really not worth the hassle to add the third additive axis.

LLoyd