Personnally I've no objections in someone adopting a POD that was proposed by someone in a thread. There must be thousands of "What if" threads on the board that have never been turned into a timeline: thus, it could be quite okay to use a POD proposed in a "What if" thread. Of course, courtesy would have the one who wish to use the POD ask the one who proposed it in case the latter is planning to make his own timeline.
There can also be some sort of negotiations: it's not uncommon for a lot of people to have different visions of how events would turn because of a POD. If, by discussing, the two people realize their timelines are going to be very different, then I think it's safe to let them use the same POD.
Now, I don't know if this thread also includes the possibility of people who wish to make a "timeline take-over": by this, I mean people who enjoyed a timeline that has died out and wish to take it over from where it stopped. I'm rather hostile to that scenario, unless the original writer of the timeline has given his approval and announced it on the board. If not, it looks more like theft...
A similar thing would be for a "multiple writer timeline": someone can't just write his own part of a timeline without the agreement of the writer(s) of the said timeline.