WHile I though that was kind of annoying at first myself...
One quick question.
Do you have to adjust the focus (or for the lazy among us (which includes me) using autofocus, does the camera have to) when you change the focal length of your lens? Of course you do. You are changing the zoom factor of the angle finder so you need to refocus.
I would hate to think how much it would cost (or how much bigger it would have to be) if it had autofocus installed.
Lastly, at least you don't have my problem. After many years of looking through a zoom 500 microscope, my eyes have a bad tendency to adjust the focus themselves to compensate for the optics, making it hard for me to know for a fact that the lenses are correctly focused for the sensor plane (I think I have mentioned multiple times that I hate the 10D/D60 focusing screen). This is one of the reasons I like the angle finder C, if I am going to have a short depth of field, where focus is critical, then I put it on 2.5x and play with it and the lens until I am confident that the lens is focused correctly. I can sometimes use the 1.25x setting, but almost never have been able to trust the camera viewfinder by itself.
I personally love the Angle finder C as it has saved me severe neck strain on occasion and it occasionally helps with focusing. I don't always use or need it, but when I do it helps a bunch. MY only complaint with it is it seems a little loose and I agree that the price seems a bit much (although I figure it is a supply demand issue). I am happy with the quality of the optics themself, so feel the price is acceptable (especially since I have it already and have gotten good use out of it.