I like union suits but this has advantages. The merino wool doesn’t itch.
You can still take advantage of the zipper to number 2 too if needed. Of course this has no top but it’s a lot easier to remove top layers as needed.
Hunting and on the job there are times you lay up being very still for hours then need to move a distance quickly
This system will reduce the time is takes you to dress appropriately so you don’t overheat.
It also could work in reverse. I usually pack in my coveralls to hunt and put them on once I arrive and cool down. The same could be accomplished easier with long Johns this way. The same on the job of going into a hide or an OP.
Not pulling off your boots in the swamp, on stickers or losing the warmth is a big deal.