Hi, Richard (and the rest of you correspondents.)
Another reason (and in my recollection, the most important) for
heating the intake manifold with the coolant was to reduce emissions
when the engine was fully warmed up. By regulating the intake charge
temperature (note that the coolant-heated version of a manifold has
much tighter regulation of the manifold temperature, because the
coolant is [presumably] at a [nearly] constant temperature) the fuel
system could be safely tuned to a leaner condition.
I recall that the Mk3 intake manifold also had the exhaust manifold
attached to it, so it would be possible to overheat the intake if
there weren't other temperature regulation means available. So the
quick heat came from the exhaust manifold (for quicker warm-up) and
final temperature came from a balance between coolant regulation and
exhaust heat.
FWIW, Near-modern vehicles used an air valve which mixed hot air from
around the exhaust manifold and ambient air to achieve a similar
result, but with MUCH better regulation. This allowed the carburettor
to operate with air of "known" characteristics. Fuel injected engines
can often get away without the heated inlet air because the regulation
of fuel delivery can be much more precise than the carburetted systems
could provide and modern sensors can measure the actual air flow more
precisely over a wider range of temperatures.
Donald.
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