Interesting thread this.
I must admit I have never took much notice in the number of hurdles/fences that are supposed to be jumped over any particular distance, I have noticed during the winter time how many obstacles are now omitted due to the state of the ground.
I ran a query using Raceform Interactive and ended up with a database of 21,000 races from 2006, it was certainly an eye opener to how many obstacles were omitted from meeting to meeting, so the subject whether or not
Market Rasen
has special dispensation to run with only 17 fences over the 3m1f course falls into insignificance.
At Catterick on the 1/1/2006 there were 3 x 2m3f hurdles run, in the 12.15 –
9
hurdles were jumped, 12.45 –
8
hurdles and the 1.20 –
10
hurdles were jumped 
I suppose that if I speed rating these races I would subtract
1lb
for each hurdle omitted 