[popo]

The "bokeh" is fine compared with mirror/reflector counterparts, which often result in awful and very distracting "donut highlights" as well as softer images and low contrast which pretty much counts them out for nature photography.  

 However, with the recent lenses from Tamron/Sigma 150-600mm producing very good results for around $1,000 with variable aperture, image stabilization and fast auto-focus,  unless you find a very cheap S/H telescope example suitable for DSLR photography, it is difficult to find a good reason to try one!

The contrast reduction for obstructed designs probably isn't significant for normal photography. It is only of concern if you're trying to pick out faint details for astronomical uses.

Also, the value of picking a telescope over a photographic lens really depends on the application. Up to 600mm for so, if you don't need a fast aperture, then photographic lenses are hard to beat. It is once you reach longer focal lengths that telescope designs really take the value crown, but of course their use and handling will be different. I've tried imaging at 6m focal length before, and that's tough at my technology level. There isn't really any gain from 4m, which does give a big difference from 2m.
 

Of the DSLR camera manufacturers, only Pentax offers a telescope lens, a 560mm f5.6. It does AF, is expensive, and not the greatest performing lens one can imagine. And pretty expensive.

What makes that a "telescope lens"? Ok, in a quick look, the length + flange distance would suggest it isn't telephoto design, but regardless I'd still consider it a lens.