Roubaix in 2009 was an extreme example of this thanks to a stretch of hub- deep mud about half way around the lap. Every class was doing it. If your wheels can't turn, you can't race. If you can't race, and the course is designed to be muddy - in northern Europe? duh! - then why bother turning up? Changes are part of 'cross. You won't see it at Koksijde or on a frozen course though.
At regional/domestic level, bike changing is very dependent on when you race. Our regional league, vets race first [1100 hours] and for 40 minutes - roughly 6/7 laps - meaning that courses are rarely chewed up and bike changes are not usual. Often they are frozen solid until the Elite/Senior practice laps at 1230-1300 hours. I would estimate 20-25% of races require changes, that's all. The Elite/Senior races are more muddy with probably 25-40% of events requiring a bike change to maintain position.
A few years down the line, it might be interesting to see how the recent rule changes will have an impact as frame and forks adjust to accommodate discs and 33cc tyres. Will the UCI legislate for technological changes by making pit stops more time punitive? Will the added weight of discs prevent wholesale shift in their direction amongst the pro's? Could/should 'cross course design evolve to factor technology at the disposal of racers?
it's quite an interesting period in cross' evolution if these rule changes stick.