Sorry for the late reply. Funnily enough, we were out for dinner with former colleagues, now friends. Yes, we'e in the UK, London based, so both our workplaces are somewhat international but have plenty of UK natives.
It's a high stress profession with long hours, client development (socialising) and travel. The last thing people want is long team lunches, team-building away days, four day corporate retreats, black tie dinners with their office mates or awkward drinks after work when you want to get home, all of which adds up to more time away from family. The people we work with, for the most part, do seem to enjoy socialising and being able to bring their families when it feels appropriate. It could just be our workplaces, I suppose, but yes, by in large everyone seems to enjoy having partners and spouses invited to meet the people they work with day in and day out in a casual non-office environment.
It started because one year we had a summer party at our house and invited people from both workplaces. Everyone had a great time (apparently) because they stayed until about 3am, word spread into other departments and people kept stopping by in both our workplaces and asking if they could be invited next summer. I talked to a several people and asked if they thought it would be nice to invite spouses and partners and it was a universal yes. Our departments have grown, it's now a lot of people, so we make sure everyone gets at least one invite in the course of a year. If someone has young children, I'd try to invite them to a Saturday afternoon barbecue type thing so they can bring them.
No expectations or pressures, though - they're being judged at work on the quality of their work based on lots of metrics and feedback from many sources that have nothing to do with coming to a lunch or party. If there are some who would rather not, I would imagine on the 2 or 3 times a year they're invited to something, they're capable of saying no.
Adding: both DH and I enjoy socialising and hosting and bringing people together in what we hope is a relaxed, fun atmosphere, and people generally react very positively to that. I'm not sure Mumsnet is always representative of most people?