All rules like this do is punish the heavy AC users, with nothing to balance it out. We already get check and speed penalties, can't hold items in the off-hand (if using a Heavy Shield), and light armor users can still usually match or better our AC without too much trouble. And then you're going to add yet another penalty on top of that???
It depends on how you implement the rule. An auto-hit on a healing surge, sure, I agree with you.
But, if the rule is:
Make an Endurance check = 10 + 1/2 level + core AC bonus (2 for leather, 8 for plate). If you make it, you're fine. If you miss by 5, you lose one healing surge. If you miss by 10, you lose two healing surges, etc. Endurance trained PCs often make the check. Others, not always. Plus it affects all armors Leather or higher.
This incentivizes players of most armors to train Endurance or CON. I find it amazing that PCs are walking around the world with 10 CONS and nothing bad ever happens to them shy of in combat. City slicker Wizards who never catch a cold in the wilderness. It's not a matter of making the game super realistic, it's a matter of creating a believable world while not penalizing one group of players too much over another. Sure, the Wizard in Cloth avoids this, but then again, he's is Cloth. Shy of taking a feat, taking a specific class feature, and/or a starting Int of 20, he typically has the lowest AC in the entire party, often 4 to 6 less than the heavily armored PCs. Plus, it incentivizes a Wizard player to take Unarmored Agility instead of Leather Armor (which might play into a given DM's worldview of how some classes should be).
It can also incentivize players to come up with different solutions like the Secure Shelter ritual when camping out.
By the way, you don't think that Wizard and Cleric and other PCs aren't penalized huge for AC before they even step out of the door? You talk a lot about how the heavy AC users are penalized, but I think that there are a lot of lightly armored penalized PCs. AC is often a much more important game element than Climbing or Stealth. Not always, but often.
From my perspective, many of the heavy AC PCs get a lot of advantages over the lightly armored ones and it's not just AC. Hit points, healing surge value, number of healing surges, AND the heavily armored PCs still can often do magical sounding things like shifting multiple squares across the battlefield, or teleporting, or auto-damaging foes. One of the least balancing aspects of 4E is that the heavily armored PCs can sometimes do many of the magical sounding things that the lightly armored PCs can do, but they still get better AC and hit points and healing surges out of the deal.
Heavily armored PCs also do not have to bump up Int or Dex every single time, even if it is not their primary ability score. That's a real significant advantage.
I had a player complain when her PC was climbing down a rope and immediately got into combat, and I told her that she didn't have her weapon out or her heavy shield prepped. To me, there are a wide variety of encounters in town where the PCs are not heavily armed and armored that cannot be played if the DM and players do not have a world view that, well, you are not wearing your 60 pound armor 24/7. You have to take a bath sometime. Sorry, but you cannot climb a wall with a weapon in one hand and a shield in the other, etc. As a DM, I enjoy having encounters that are not just "go to dungeon, kill monster, win". House rules like these can open up a lot of possibilities that don't often happen in games that play inside the box of the core rules only by incentivizing players to actually roleplay the PC as if it really was a person with 60 pounds of armor on his back.
