For the record, I am not in the camp that brings my own PFD onto a liveaboard or day boat. My guess (and hope!) is that no one had an issue. However, based on the OP's experience on where they have seen PFDs store and located, that is what warranted the idea to them to begin with. If that helps the OP sleep better at night, I say go for it. However, I suppose you could say I wouldn't know until it became a problem and this is a problem that I would want to prevent or never find out. That is easily mitigated by bringing your own PFD. It is a personal risk assessment. For what it's worth, if someone chooses to bring their own PFD on the boat, it's fine by me. I don't pass any judgement on them, or anyone that travels with a CO detector, PLB, etc.
Someone won't need it until they need it. The next question is, where do we draw the line when it comes to "just in case" items?
Your last sentence is exactly the question. Do you sacrifice precious luggage weight and space for an item that might be needed once in (to make up a number) 10,000 liveaboard trips? There are a lot of boats out there every single day, meaning maybe a thousand people worldwide at any given time, all year, every year. Even if on average, there is one LOB abandon-ship per year somewhere in the world, are PFDs even an issue? I like to quantify risks a little more than this before I bring along safety gear.
I’ve been on those overloaded Indonesian ferries where they basically shut everyone inside, with no obvious escape route except the way we were herded in. Now that was scary.