When Canon released the first 1D, APS-H was simply the largest sensor they could get away with, economically. They followed it up with the 1Ds which was full frame. However the 1Ds was slower than the 1D, and offered less reach with telephoto lenses, so was less popular with sports and wildlife photographers. For this reason Canon chose to continue offering a faster, lower resolution 1.3× crop body in the 1D line.

For whatever reason, Canon decided to end the 1D line with the introduction of the 1DX. They were able to exceed the speed of the last 1D whilst improving resolution (slightly) and offering a full frame sensor. Even so the decision wasn't universally popular, especially with photographers that appreciated the extra reach from the 1D crop factor.

Would be an interested to see an answer from a late-model 1D-user!

I had a 1DmkIV for 2 years. I was a 5D user but was never fully satisfied with either the speed or more importantly the autofocus with the outer points. I waited for the 5DmkII to come out and when it did I was very disappointed that they addressed neither the speed or the AF performance.

The 1DmkIII was a 2MP upgrade from its 8MP predecessor, whereas the 1DIV jumped to 16MP. This prompted me to switch from full frame to APS-H, as I could still get large prints if I needed to. I found no fault with the camera in terms of its AF, speed, or features, but I had to convince myself I could live with the crop factor.

However over time I began to find the crop factor annoying. Ultra wide angle lens options were very limited, I ended up using the EF-S 10-22 which I modified to fit the EF mount and was usable without vignetting from 13-22mm. The only problem with this was that the mirror would hit the back of the lens if I accidentally zoomed right out. I also missed the 85mm FOV, my 85mm was a bit too long and my 50mm was a bit too wide.

In summary I don't think there's anything magic about the 1.3× crop, unless you need the reach I think it can be quite annoying.

When the 5DmkIII came out with top-of-the-line AF and 6.5 fps, the decision became clear and I sold the 1DmkIV. Fortunately they had become sought after since their discontinuation so I sold it for a fraction less that I had paid for it!