Well, actually your description is how Dominion is supposed to work, as described by Donald before. All below-the-line abilities potentially trigger all the time, which is why they either have to have their scope defined ("while this is in play") or be restricted by when they can be resolved ("reveal this from your hand"). Above-the-line abilities are limited by their implied scope "when you play this", Events imply "when you buy this", Landmarks have unlimited scope, etc.
But, the difference is the model of the "triggering window":
Model 1: Some things trigger, and others might trigger while we're in the window.
Model 2: The one you describe: Everything triggers at the same time and never again.
Dominion is apparently using model 1. But it's not only "while this is in play" abilities that would be affected by adopting model 2. Ghost playing Ratcatcher would indeed work the same, but I don't think Ghost playing Hireling or Prince would, since those didn't trigger before they were played by Ghost. I can't find any other differences though.
So... let's use Groundskeeper as an example of a "while this is in play" ability.
When I'm thinking more about it, it seems like in order for it to work according to the ruling, it's equivalent to "when you gain a Victory card, if this is in play". I mean, I can't find any difference between how this would work and how Groundskeeper is supposed to work. In that case it's more like Basilica than Guildhall.
Basilica - When you buy an Action card, if you have $2 or more left...
Guildhall - When you gain a Treasure...
Only the underlined phrase is the trigger. Basilica first triggers, then we resolve when-buy abilities; and this could make a difference because by the time we get to "if you..." things could have changed. On the other hand, Guildhall only triggers if you gain a Treasure, and we don't check the gained card again. It could be a non-Treasure by the time we resolve Guildhall, but Guildhall doesn't care.
So then Groundskeeper is functionally like this: (A:) When you gain a Victory card, if this is in play...
Again, only the underlined phrase is the trigger. Viewing it like this, it actually also works in model 2 too, just like Basilica.
We can instead view it like this: (B:) When you gain a Victory card with this in play...
(This is similar to how Urchin is phrased, but we have nothing to compare with there, because nothing interesting can happen in that window.)
In this case it really seems like it would work like Guildhall. That means it wouldn't work according to the ruling, not in model 1 nor in model 2.
So how do we have to interpret it for it to work in model 1 but not in model 2? I guess something like this:
(C:) When this is in play, when you gain a Victory card... which is functionally the same as When you gain a Victory card, when this is in play...
Here we're treating "when this is in play" as a trigger (which is what I alluded to in an earlier post in this thread), giving the card two triggers that both need to happen. It's a bit weird to say that "while this is in play" (the actual card text) is a trigger though; it doesn't seem like something that happens in an instant like other triggers.
So I guess Donald could think either (A) or (C) here. I have to say that I'm starting to think that (B) is the most natural interpretation though, which would agree with your feelings.