I can't think off the top of my head why you wouldn't want strict vars in code you intend to maintain, but I'm sure there is a justification sometimes.
I would care about proper scoping of variables (which also
implies using lexicals where lexicals ought to be used).
"use strict" doesn't enforce scoping. It's not a magical wand.
It's not a requirement to have proper scoping either. A program doesn't become bad (or harder to maintain) if you
remove the "use strict" line for a good maintainable program,
neither does a bad program become good by the introduction
if a "use strict" line.
"use strict" is an aid but it's neither necessary nor sufficient to write good programs. I'd be hestitant to work for anyone who focusses on details, forgetting the overall