Long term value of PTS cannot be defined just by the value of PTS. Remember, PTS gives you a share in other exciting technologies. For instance, it's possible that a year from now, PTS is trading at 0.01BTC, around the current rate. Does that mean that was the long term rate of PTS? No, because in a year, most likely, you'll have Bitshares out and perhaps they are trading at 0.1BTC.
The value of PTS depends on whether you hold them before or after another Invictus DAC is released. Each time such a DAC is released, theoretically at least, you should see a price drop.