I don't get this...
I understand the mathematics, but I don't understand how this makes sense in a real world example. You can't expect to gain $25 for switching, because you have a 0% chance of gaining $25. You either gain $100, or lose $50. If we perform a long series of envelope switching exercises, and I switch every time, then I'll average a $25 gain over the duration. But that's not what we're doing. On a one-time-shot basis, you have just a 50/50 chance of coming away with more money. On a risk vs. reward basis, you're an idiot for not switching, though.
I'll tell you why I would switch. I've just been given $100. Regardless of what I do, I'm coming away with more money than I showed up with, because there's some moron giving away free money as long as you play some weird envelope game with him. Might as well go for the $200.