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Sasup,
The reasons people don't like longer focal ratio Newtonians are:
a) the big tube and length makes them awkward for an amateur using home tools to make a stable mount such that it is reasonably steady in even a light breeze. A Dob is the easiest; on anything else (german equatorial) a long Newtonian will be VERY sensitive to wind and the only real solution is to house it in a dome to keep the wind off.
There is a rule-of-thumb that for a Newt on a German equatorial, the diameter of the worm drives must be at least half the aperture of the scope otherwise the geartrain won't be sufficiently rigid to stand the torque applied by a large tube in any breeze, and hold it steady.
For this reason once you get up to say 20 cm aperture, you'll find some using a 20cm f/4 or f/5 for imaging, but no-one would attempt imaging on a portable Newtonian at f/7 or f/8 - it would be impossible without a dome for shelter.
By way of example, a german equatorial mount suitable for imaging with say a 12" f/7 would have to be one of the larger Astrophysics or Mathis mounts and the cost will exceed $15,000 just for the mount. For this reason the larger SCT's from Meade/Celestron start to look quite attractive if you want a long focal length.
b) On an equatorial mount the eyepiece is often in very awkward places unless you have some way of rotating the tube. Hence tube rings... Dobs are by far the most convenient if you're a visual observer.
c) Once you get up to 25-30 cm aperture, anything longer than f/7 means climbing ladders to reach the eyepiece. Dangerous in the dark and even more so on uneven ground.
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