How do your power adapters "wear out"? Ive got a bunch of various MagSafe adapters over the years and, except for one that melted and Apple replaced, they all work and look fine so I'm wondering what is happening to yours.
The wire themselves wear out. The rubber coating gives way over time due to the stress of how the wire gets angled. This is usually right at the end of the wire where it meets the MBP (or where it meets the power brick). Sometimes the rubber gives and the wire becomes exposed and eventually fails to complete a circuit. I've seen this happen to a few friends and family members. But with me, the wire itself seems to run in to issue before the rubber coating gives. Especially on the L design. Kind of like a faulty connection that you might get on earbud or headphone wires.
Just google search "frayed/worn out macbook charger" if you want a visual image or some blog testimonies. It isn't unique to Macbook chargers. Lots of cables will run in to this kind of wear and tear. Usually, charging cables are thicker than the ones on Macbooks, so the heat may be a factor (given that the issue is usually near the tip or where it meets the brick). I've seen it in USBs, mice, keyboards, some game console controllers as well. Not all necessarily mine, mind you.
I've been in school, so I don't always access my Macbook from the same spot on the same desk. Sofas, my bed, coffee shop table, libraries, class rooms, etc. In a few of these environments, the cable might be put at an extreme angle. Like in a classroom where I can only access a wall charger that is on my right. MBP always have magsafe on the left. So the cable gets angled and stretched, and thusly undergoes stress. With time, that can wear out the cable, and they'll stop charging your battery because they can't complete a circuit. At which point, I just replace them.
I don't always have a choice in the situation, so its just something that I've accepted over time, like tire tread wear. I'm not complaining about it, since I know I put my MBP in that position at times. I simply noticed that my Ls didn't seem to last as long. Which I found odd, because they should be able to reach that right side socket a little easier. I have noticed that, unlike the original T, they get stressed out by reaching for a left. The L shape's design simply seems stiffer and more apt to run in to the issue. And yes, I usually use the extension cord that comes included with every MBP.