Ivor, it's true that if you need perfectly round out of focus highlights, than many(but not all) prime lenses can give you that. In situations like the above many lenses would have produced an outline or slightly irregular looking shape to the out of focus highlights. However if you simply need a smooth out of focus background then using as long focal length as possible is even more important. You also need to shoot as wide open as you can (use as low f-stop as possible) and select a smooth and distant background. My background in this photo is out of focus canopy, probably 60 feet or so away and the highlights come from the setting sun. This is a bit more challenging than for instance using grass as a backdrop. As grass is much smoother it can be much closer to your subject and still be nicely out of focus.
So all together I wouldn't say that it's a must to use a prime lens for good bokeh, but it can help in some situations, all depending on your needs.
Sorry if I explained things that you're already familiar with!
