I don't know that I've ever researched this - but I did have two PSU's of different vendors when pushed to their limit become weaker over time.
A sparkle 400watt PSU, that was pretty much maxed out started to become unstable at less and less an overclock on my old p4 3.2Ghz system with an 6800GT and mutliple hard-drives. Soon it couldn't run anything but stock. I bought a new PSU and my overclock was stably obtainable again - In fact higher levels of overclock. Then the same thing happened with an Xclio 450bl PSU on another system.
It's weird, but it can and does happen. All electronics will 'deginerate' a bit over time. Capacitors leak, voltage fluctuations at the wall take their toll, etc.
Think about it in a car terminology. Does a 100,000 mile corvette have as much power as a brand new corvette? Even if we are talking about a very minimal amount of loss --- when the power supply levels are so close to being maxxed - it matters.
Hmm, that's interesting about your real world scenarios of weakening PSUs. I never had such a problem, but I guess I wasn't pushing the system too hard. I thought of PSUs (in terms of weakening or dying) like light bulbs. They rarely dim significantly before dying. They work fine and then just die one day on without much or any warning at all.
On the topic of PSUs, does anyone know who makes the ones HP sells in their Slimlines?