Its called Angelshares.
The big pro for PTS is that they are liquid. You can decide which DACs are of interest to you. If somewhere along the road something pops up that you don't have interest in you can sell your PTS before the snapshot, and rebuy a day later. I believe these price movements to become less and less extreme in the future though.
I think the mass sell-off was because at the moment BTS is the only more or less tangible DAC at the moment, and I3's vedette.
With the last snapshot BTER credited users accounts with Bitshares, no there was no need to move the PTS off the exchange. Don't know if they'll keep doing that in the future though.