With a good Barlow Lens, proper choice of eyepiece focal lengths can effectively double the number of oculars an observer has to choose from. Collimation is important for good imaging, and critical for planetary observations. Alan pointed out that shorter focal-ratio newtonians can be more difficult to keep collimated. My experience as an SCT user is basically collimation-free. Another friend suggested that I check against a fourth magnitude star.
Contrast is important for planetary observers.
It can be important for observing galaxies, as well as emission, planetary and reflection nebulae. It is certainly offset by the increased resolution gained by larger aperture that is prohibitive in the unobstructed refractor and may only be obtained at considerably greater expense. Of course central obstructions in short focal ratios are minor compared with the optical system and supporting infrastructure. Without them you have a poor telescope, and contrast is the least of your worries. But, there is nothing wrong with increasing the performance of a well-configured system.