How does this prove custom objects are useful? :)
I don't know about you but I usually create materials in separate scene with a studio lighting setup and for this I tend to use some more complicated object than a sphere to better understand how the light will reflect and refract.
With custom preview I can use the same object for previewing and hopefully need to click render fewer times. :)
A question vaguely related to this thread: Is it possible to adjust the preview scale like you can with the default preview from Compact Material Editor -> Options -> Render Sample Size ?
Doesn't seem to be working with custom MTL preview.