Colin's results not withstanding, it really depends on how deep and well resolved you want your data.
If you shoot at 500mm to 1100mm, then accurate tracking is a no-brainer for many mounts. But when you shoot at 3400mm on a good night, i.e. FWHM's under 1.8 arc sec, all I can say is: lots of luck with absolute encoders.
I constantly see stars move in RA
and Dec....due to no fault of the mount, just the atmosphere doing its thing.
I have have yet to see an unguided mount produce tighter stars than a guided
mount. If I use an AO then I see up to a 30% improvement in FWHM's
I'll also be putting my money where my mouth is, as my next mount will likely be a Paramount Taurus.
Being able to image uninterrupted to and through the zenith is something I should have done years ago.
