[Brightcolours]

You've already answered your own question, innit? 

 

"Especially since the constant f4 is not always very useful for an UWA for APS-C."

For landscape shots, you would still stop down some, but if you're interested in employing it for  general use, it's a blessing over the f/4.5-5.6 designs. The aperture is constant to boot. I used a Tokina 12-24/4 in the days of yore and it was a great experience (sadly I had to part with it in anticipation of stepping up to APS-H)."

"Not always very useful" is answering what, exactly?

F4 compared to f5.6 is not very useful a difference on APS-C with an UWA zoom, in my opinion. You are not gaining a lot of shallow DOF (to say it mildly) and exposure time wise it is still an UWA anyway.

 

I too have had the crappy put together Tokina 12-24mm f4 (which would have worked just fine on that APS-H camera) and do not get why its constant f4 would be of any advantage.

 

And that is not worth €600 or €731 to me..