David Richerby wrote:

Do you have any guess (or, shock!, data) on how often errors occur in
WC games that an engine (given reasonable time) would score down by
say 100cp?

I will say, that in my own practical experience, running through games.
That in the same, and not unusual positions, that Fritz 8,9,and 10
have evaluated positions over 100cp different than Rybka 2.3.1 And
that different moves have been suggested.

That alone should provide enough of a question as to the results here.
The fact is that we don't know when the engines will be strong enough to
represent the "truth".

I will say that I do not use Crafty for day-to-day analysis so I don't
have an opinion other than that you need to remember in ELO that the
difference between 2500 and 2800 is vast, and the difference between
2800 and ~ 3100 is as vast. It is not 10% better, it is closer to think
of it as TWICE as good. Or more likely to win MOST of the time. It is
a HUGE difference.