I regularly "social" dance with my partner, but we intentionally pick events where neither of us know many people so that we can dance almost exclusively with each other. We're obviously intending those events to be competition practice, and not really trying to be social. It's just a convenient time where we have music to dance to.
There's a particular social I have gone to for a long time for actual social dancing, long before I had a dance partner (I started as a purely social dancer). My partner doesn't usually go to that, but I've had the same thoughts/questions you pose if she were to go more often. I feel it would be rude for me to not spend at least 1/4-1/3 of the time dancing with friends instead of only my DP. There are a few people who go to that same social with their partners and dance mostly with their partners, but they do dance with others as well, and I never hear any complaints from other people about it. I would probably tend to ask my DP to dance a lot because I really like dancing with her, which is a big part of why we became dance partners.
Typically we'll dance the dances we're preparing for competition. Those occasions are our best time to work on lead/follow skills, and in many dances we don't have routines yet, so usually we do lead/follow. In a couple dances we only recently have routines, so we also practice trying to remember our routines at the speed of the music (with a dance like Jive that's sometimes a challenge for us). But we also will continue dancing if we mess up our routines.
We do tend to sit out the dances we're not competing in, mostly to rest, but we'll sometimes dance other styles that we both know (like salsa) just for fun, so that dancing with each other does not always feel like a high pressure thing.