Is there a particular advantage of the ES fitting over the bayonet
fitting in these sorts of applications?

No. There are several significant disadvantages to ES bulbs over BC
(polarity reversal safety and the bulbs stick, meaning you sometimes need to
replace the entire fitting, not just the bulb).

They are becoming more common simply because some markets do not have good
availability of BC bulbs and they only want to produce one version of their
product.

Christian.