Baby cinema isn't an option everywhere. It's not at my local cinema, and it would require a three hour round trip to get there, which wouldn't have been doable first thing (which is when they all are, annoyingly).
I think people know their own children, and if they have the sort of baby that would be OK with it, and would be unlikely to disturb others, then I don't see the issue.
Sitting right by the exit or loos, so you don't have to clambering over everyone, in a constant state of ready to move, you could nip out at the first stir. There'd never even be a cry.
Not all babies cry or make noise when young, and some are so predictable, that you could be confident that they would make zero noise in a cinema.
I wouldn't go now with a nine month old, but I'd have done it when she was three months. I take her to church most weeks, and the vast majority of time (when younger, less so now) , she hasn't uttered, a single noise, sleeping through drums etc.
If you've got a baby that can be unsettled and does cry, then you should give the cinema a miss, but a napping baby that's in a sling, with a quick getaway planned, won't disturb anyone.
And if people don't like it just because they had to get a babysitter, then I don't think that's a very good reason to object.