make maintainer-clean and config.cache

In side-note in another thread Tom pointed out the speed improvements of
using an autoconf cache when re-building, which sounded nice to me as
config takes an annoyingly long time and is not parallelized.

But the config.cache files gets deleted by make maintainer-clean. Doesn't
that mostly defeat the purpose of having a cache? Am I doing something
wrong here, or just thinking about it wrong?

time ./configure --config-cache > /dev/null
real 0m21.538s

time ./configure --config-cache > /dev/null
real 0m3.425s

make maintainer-clean > /dev/null ;
## presumably git checkout a new commit here
time ./configure --config-cache > /dev/null
real 0m21.260s

Cheers,

Jeff