What I seem to remember (it was so long ago) was that you'd open the top part for some reason, realize you had to either go outside or let someone in (the reason you had opened the top). Then you had to close it again, hitch them together, and reopen. Sometimes you wouldn't hook them together quite correctly, and you'd go to open the door to go out, and either only the top half or the bottom half opened, and if you were moving fast you'd walk into whatever half was still closed. Sometimes if you were in a hurry and the top was open, you'd just quick open the bottom to get out, but then you'd have these 2 halves flopping around independently. At times like that, it was inevitable that your arms were full and you couldn't just push the door closed with your foot or hip, but had to go put every down and go back and wrangle the halves back together.
Maybe that doesn't sound like a lot, but if it's a door you use a lot, it just doesn't seem like it's worth the effort.
Me? I like to eliminate steps, not increase them.
Thinking about it, it reminds me of my confusion over why these tiny apartments and houses that have special designs are getting so much press. You know, you can live in a small space because you pull out this and you have a table, put it back and pull something else out and it's a bed. Move a wall and there's your toilet. If I lived in something like that it would be a hot mess within 2 days...well, maybe 2 weeks.
There's a reason these doors aren't all that popular, except in barns, or maybe day care centers and such where little ones could get out a totally open door. Places where they aren't opened and closed much.