Bill, springs wear out regardless of what they are made of, used for, or their thickness. This is a fact of engineering/physics. They more compression and longer comp time, the quicker their properties change. I WILL grant you that the change may be so small that it nearly immeasurable over the magazine's useful life. However, without the data to back it up, I like to play it safe. The practice extends their lives by 33% (theoretically). Obviously the clear solution is to buy mags solely for SD, which I will soon be doing.
"When a strong man, fully armed, guards his own palace, his goods are safe."