The wristband system
should work fine for EMH and parties, since the bands are visible and don't require time-consuming interaction between CMs and guests. A quick glance, and they see it (or don't). The only deal-breaker is lazy/incompetent CMs.
During rain or cold, it makes checking bands harder. When ponchos or sleeves cover the wristbands, CMs would have to stop almost everyone show their band.
We went to a P&P party where it MONSOONED all night. Most people had on ponchos or jackets, and we saw TONS of people without wristbands throughout the night, including on rides.
It's easy to understand how that happens. It's late, and tired, surly guests who've been slogging through downpours all night might balk at pulling up their sleeves at every ride. If enough of them act huffy toward a CM, the CM might just make things easier on him/herself by not bothering to ask.
Hmm... maybe they should just stamp everyone's forehead with that invisible UV ink they stamp your hand with, then install black lights at each ride so we all glow in the dark.