a good smooth dive with a new buddy is about their quality as a diver (and as a person), and about the two of us being on the same page in terms of goals for the dive. Fundies is a great way of getting there, but it isn't the only way.
I've done any number of good dives with non DIR-trained people. A good diver is a good diver. But there is an extra quality of relaxation when I know my buddy's gear like my own, and I also know what that person is likely to do in a lot of situations. And I know what to expect them to be able to do. It's just wonderfully seamless.