How often do you actually use such lenses? As it is, the 17mm shift lens is so wide that many architectural photographers seem to use it very sparingly. As you note, at these extremes the stretching effect at the edges gets especially hard to control, and there is the extreme scale distortion to consider as well.
If it is not too much trouble, could you show us a few examples of photos you have done with these really extreme wideangles and architectural subject matter?
I used it in the last weeks surprisingly often, although before long time nearly not. It is very good to have such lens in the bag, there are situations where you cant capture a building with wider lenses for having little space ...
In the project of the attached images i think i used it in nearly 30% of all the 25 shots i took. This was two weeks ago in Japan. last week i photographed in Doha the new convention center. There also the 12mm was very usefull in several shots.
Reg. the stretched sides. Either take care whats in the frame or - if this isnt possible - edit the sides in PS. copy and layer them and push them together a little bit till the image looks more natural.