I'd estimate I've taken ~50 or so flights with my partner where we weren't a last minute booking due to a cancellation or missed connection, and we've never sat apart. I'm not sure I've ever seen the assigned seats at check-in be apart either.

Most of our flights are trans con and after a couple incidents of having unpleasant seat mates in Y in an A320/737 we did a few trips in PY which were great, but now we just book a 777 and pay for advance seat selection and grab the 2 seat section at the back which gets us 95% of what we wanted out of PY for $30 each. Shorter flights out of YVR there's usually an option for an RJ and we have so far always been placed together so don't bother paying for seats in advance.