From my experience a dual-speed focuser is almost essential for imaging. Just the slightest tweak can be the difference between clear focus and just slightly off focus - using a Bahtinov Mask as a guide. The sort of difference this has on an image is almost undetectable to the human eye, but then for imaging the exposure may be 10 or 15 minutes and any out-of-focus becomes painfully evident with fine detail.
A dual-speed focuser will always help attain the fine point of focus, but this also depends on the magnification - the nearer to 180x plus, the more critical it becomes. But then other factors become relevant: the seeing; the transparency; focal ratio; the viewers eyesight; the quality of the eyepiece; collimation; chromatic aberration; visual acclimatisation and not the least - expectation.
So for viewing a dual speed focuser may not be the first item to consider. A high quality eyepiece and a well collimated 'scope (in good atmospheric conditions) would be better places to look for improvement - in my opinion. But then I'm an imager - so what do I know!
Yours aye - Cap'n
And if the dam breaks open many years too soon; And if there is no room upon the hill
And if your head explodes with dark forbodings too; I'll see you on The Dark Side of the Moon
My Flickr site for all my astro images - and other stuff, if you'd care to take a look!