I've found it matters what character creation method is used. With Standard Array or Point Buy, Medium Armor is very niche, since most character will either dump Dex or max it out. With Rolled Scores, Medium Armor becomes much, much better, because many classes that do not get Heavy Armor aren't Dex heavy by default.
In the Thule game I play in, we have 2 Medium Armor, 2 Light Armor, and 2 no-armor. My Greyhawk game has 2 Heavy, 1 Medium, 2 Light, and 1 no-armor. Both of these games use rolled characters, and so we see a pretty good mix of armor types.
I totally agree with the 3E failure of one "best" type of armor for each proficiency, and really wish there was a better way of doing it.
In the Thule game I play in, we have 2 Medium Armor, 2 Light Armor, and 2 no-armor. My Greyhawk game has 2 Heavy, 1 Medium, 2 Light, and 1 no-armor. Both of these games use rolled characters, and so we see a pretty good mix of armor types.
I totally agree with the 3E failure of one "best" type of armor for each proficiency, and really wish there was a better way of doing it.