Usually the reason is the "slow down" portion is very small, and it's confusing to shift down the actual speed limit for a 200 foot stretch of road then increase it again.

There are signs for that. Advisory speeds that don't change the actual limit. https://wisconsindot.gov/PublishingImages/doing-bus/local-go...

Much better to be specific than a vague "slow down". There's a road near me with two tight turns a couple blocks apart. One advises 25mph and the other advises 10mph.

FWIW, it seems less confusing to me than longer speed limits, but with "Slow Down".

Except we do that all the time in school zones... normally 35+, but from 7am-9am and again from 2pm-4pm the limit drops to 25mph (which is still to fast if the kids are actually crossing the street or walking alongside en masse).