I asked about this myself a while back. Jesse and Carl suggested that there is a workaround: Configure your new-connected-record form so it is hidden if a connected record already exists.
Enforcing cardinality: possible? - Help Center / Get Answers - Knack Community Forum
Seems like a satisfactory workaround.
I have used one-to-one relationships now and then in the past, in FileMaker. I did it for different reasons. Sometimes it made it easier to control access to data by users with different levels of privilege. Say I had a table named Employees. And say further that a lot of the info about employees is okay for access by all users, but some of the data (personal evaluations, perhaps) is intended to be access only by supervisors. And in the more distant past, I used one-to-one for reasons having to do with storage. In one app that I recall, the secondary connected table was in a secondary file which contained nothing but large image files (plus of course a key field that allowed the link to the main table in the other file). Backup options in FileMaker are on a per-file basis. So by putting the images in a separate file, I could configure that file so it wasn’t getting backed up so often.
But I haven’t used one-to-one relationships for a few years now.
I do agree that, if Knack’s relationship set-up dialog OFFERS a one-to-one option, it ought to be enforced. Otherwise, it’s like this conversation at a restaurant:
WAITER: Chateaubriand and pommes frites. Very good sir! And will you be having wine as well?
ME: Yes, wine sounds good. House red, please.
WAITER: I’m sorry, we don’t actually serve wine.