Having use_math: true in _config.yml. by default would be “one less thing” for people to need to remember to do in order to get started posting their work.
(As I just forgot to do, when setting up a new version of my blog.)
Is there a reason to keep math support disabled by default? Such as, is there a significant performance hit such that users who don’t want math support would be inconvenienced by having it enabled by default?
Yes I think it’s best to turn it off, because only a very small proportion of blogs use equations, and it doesn’t make sense for every blog to have the js loaded if not used.
Also, folks that can write LaTeX can probably change a config setting