Isn't this the big advantage of open source and Javascript in general? Even if the creator abandons it, you should have enough development resources to at least maintain status quo.
They chose YUI because of the current state of YUI and not because they believe on YUI future features anyway. So Yahoo bug fixes and security patches should be good enough for Wells Fargo for at least 1 year, maybe 3.
That's not quite right. The advantage is that you (and others) are ALLOWED to do what you want with it (within license limits), but there's no guarantee that you have the resources to make use of that right. Even maintaining the status quo might require more resources than you have as the world evolves and bit rot sets in. It is nice, though, that open source leaves open the possibility that some other party can pick up the ball if the original developers drop it and you can't do it yourself.
Yeah it's the pro but the con (IMO) is that if you're the last active contributor then it has effectively become a custom framework... which nobody likes to support.