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Desireable for what? Are you only considering refractors? Focuser rotation can be a problem when imaging as it can affect the collimation due to inprecise manufacturing. Moonlight offer easy focuser collimation but It's not easy on a refractor without a good laser and target over the objectve - not something you want to do whenever you'd like to rotate - better to use the tube rings to rotate the whole thing.
Dual speed focuser, with a brake, is important to imagers.
Reconfiguring the back end for imaging may require extensions tubes and removal of diagonal.
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