It's not so much that not having one is a problem, but that having one is sometimes an improvement.
How best to handle a Brompton when you're not riding it is one of those things that varies a lot from person to person (bike spec will make a difference too, vis all-up weight and easy wheel configuration): If you're tall it's much easier to carry as a folded package, as it clears the ground with your arm straight. If you struggle to lift the weight it's *much* easier to keep it unfolded and wheel it, and if you've got to carry it up and down steps, you can hook the saddle nose over your shoulder. But that only works properly if the rear triangle doesn't flop.
Similarly, whether the rear triangle is in danger of folding under on hard braking or speed bumps is going to depend a lot on how you ride it. It's not something I've experienced.
And of course, if speed of folding (or frequent use of the first-stage fold as a stand) is important, not having to reach down to release a clip is one less thing to do.
I like the way the official Brompton clip gives you the option of either behaviour, and that you can swap between them without tools. Most of the time it doesn't matter to me - I unfold the bike, ride it to somewhere, and fold it again, and having the clip enabled just adds a small amount of faff. But if I'm going to have to deal with steps, the clip is really helpful.
