More like a resurgence of an old trend I think.
A set of 9's is a good addition to the collection for those that can afford and/or keep multiple sets. For me, a decent set of 12's (or even 13's) is more versatile as a single set. I personally prefer an extra set of 12's with a softer action as a second set rather than 9s.... more useful for fishing the types of waters I fish.
It's only recently I've considered a set of 9s for one particular water I have a ticket for. It's a bit of a maze, overgrown, and requires boat work to land them from several swims. Nine's aint essential, but they'd help. Struggling to justify the spend on a non-essential though.