I think it should continue playing every night for the following reasons:
1. Some people only stay for a few days.
2. The weather may prevent people from seeing it the other days.
3. We're usually somewhere else (the parks, a restaurant) on most nights.
4. It probably costs very little to put it on every night. (I hear Wishes costs $10,000 a night and Illuminations costs $30,000 every night; this, at most, is probably under $500.)
5. A once a week schedule will vastly inflate the per show cost. Now the boats, the upkeep, and the people who keep it going will do all their work for only one night.
Therefore, I must respectfully disagree with Rooniedoo and anyone else who thinks it shouldn't play every night. When we see it, it's serendipity. Maybe we're exhausted from a long day at the parks. Maybe the weather disrupted our park hopping schedule. Maybe one of the kids had too much and just wanted to go back to the hotel. Maybe the parks were ridiculously crowded or the heat or cold were just too much. Maybe someone got a little sick during the day. Maybe we're having an early day tomorrow and we have to go back to the hotel early. Either way, if it only played one night out of seven, we'd probably miss it.
As many of you said, it's usually a surprise to suddenly hear it. You're going back to the hotel. Or you hear it from your room. Or you're in the resort pool. Or you're out waiting for the fireworks. It's just that little extra, that relatively cheap thing that Disney does to add a splash of fun at the end of the day. To me, it ranks right up there with my kids getting the occasional free stickers, the free button for celebrating a birthday or anniversary, the big Mickey hands waving good bye as you leave Magic Kingdom at night, and the mid-afternoon hula lessons outside the bar in the Polynesian. They're not why we go, but they sure add a bit of magic when you happen upon them.