I wouldn’t say a buckle at the zipper bottom is the consensus – all tents put some horizontal strain on the zipper and mid style tents don’t uniquely put more strain on the zipper than other tent designs, yet very few tents provide a buckle here. MLD is one of the few companies doing this as far as I am aware.
I don’t think a buckle is necessary for a backpacking application. It adds weight and is a hassle to use – especially from the inside and for virtually all folks it isn’t needed. When I owned MLD mids I never used it.
Consider that many companies are using much weaker #3 zippers (versus #5 in the X-Mid) on tents that would have similar zipper strain (e.g. Locus Gear Khufu) and yet failures of even those zippers are incredibly rare. I would say the vast majority of lightweight tents use unprotected #3’s (MSR, Big Agnes etc) which are far weaker. Of the companies that use #5’s (e.g. TarpTent) failures are nearly unheard of. By far the most common zipper failure is the sliders wearing out on a #3 zipper, not the zipper being torn open by high winds.
So I think an unbuckled #3 zipper is a little bit vulnerable but a reasonable choice, an unbuckled #5 zipper (like the X-Mid) can be considered solid, and then a #5 zipper + buckle would be in the realm of super beefy/overkill. A buckle would be good for those folks that like to test their tent on the summit of Mt. Washington in 80 mph winds, or those crazy hikers in Scotland that keep getting blown off the mountains.
It’s also worth mentioning that in some rare extreme wind event you could stake down both sides of the door and pull those stakes out in a opposing directions to hold the bottom of the door closed, so in some unexpected windstorm you could still support the zipper.
One day I’d like to do a beefier/fully featured version of the X-Mid (“X-Mid Alpine”) and this might have main zipper buckles. I’m still not entirely sold though because of the fiddling.
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