Sitting down with a zipped up leather.

[Edward]

Every leather jacket should be made with a two way zip

It works well for some, though I've never felt the need on any jacket that stops shy of the top of my Pelvis. I think the only jacket I had a double zip on (that I can remember.... pretty sure it had one...) was a Barbour International, about fifteen odd years back. That ort of length it was handy.

The other option is to do as here:

...and have the zip stop an inch or two above the hem to allow for this issue. Obviously puts a limit on how long overall the jacket can be without looking silly....

The Aero Hudson is a slightly longer style that takes this second approach:

The problem is almost every makers motorcycle jacket is too long.
Also, in this vicinity a jacket that you can’t wear zipped up isn’t very practical.

It's become a lot more common since the late 70s. In the UK & Ireland it started, best as I can make out, in and around the late 70s, with a particular influence being the Lewis Supermonza. Longer cut jackets to cover the small of the back on a crouched-forward racing-style bike, with a more aggressive riding stance than a Bonneville or a Thruxton-style with the low down, clip handlebars. Those same longer jackets start to be more of a pest if you ride something with a more sit-up position (the average Harley likely being the most polar opposite).

Aesthetically the problem comes in when retro looks become popular again, and then you have various companies trying to make a vintage-looking jacket, but keeping it too long without sympathetically altering the design, so it all looks just off. (Have you ever seen a Perfecto knock-off with a 29" back? Truly an abomination.) I think designs that are made too long for whatever reason (all too often, these days, so they don't leave a large exposure of shirt at the bottom, given so many people are so resistant to wearing trousers with a waistband that sits on the actual waist, rather than our to six inches south of it) are never going to work well, irrespective of zipper arrangement...

Beyond a riding jacket, though, if I wanted to sit down in a slightly longer jacket a lot, a double zip or a button front is the answer. I my wool overcoats I've been living in this Winter, I just leave the bottom button or two undone if I'm headed for the tube, the easier to access my travelcard in my trouser pocket and to sit down, if desired (or, indeed, an option at all in rush hour).

 

[Boulderunner]

Even if a jacket stops short of my hips unless you have bizarre posture and never curl your abdomen you will still need to bend the front of your jacket. Imagine sitting down on a motorcycle and hunching forward for aerodynamics but then not allowing your jacket to bend as well. You would get hit in the Face with the collar and it would look really odd. I see a lot of jackets that actually make people look a little fatter/heavier because they refuse to bend the front of the jacket and when they zip it up the flat front pushes out and looks like a giant gut. At least forming and molding the bottom of the jacket so it bends in like a person doing a sit up it actually makes the bottom of the Jacket look more slim and follows the body more closely.

 

[MrProper]

I like two-way zips.
1. My jackets are long enough that they bunch up when I sit down if they are closed.
2. When going to the loo, belts/buttons are easier to handle if the closed jacket is not in the way.
3. I find it visually appealing on some jackets when the lower part/zip is 1-2 inches open. It's like leaving the bottom button open on a trucker jacket.
Points 1 and 2 can of course be solved by simply opening the jacket completely.
Point 3, on the other hand, can only be solved if the zip ends well above the bottom edge of the jacket.

 

[TLW '90]

My only jacket that's uncomfortable to sit down in while zipped has a 2 way zipper, so I can just unzip it a little from the bottom.