What would be the incentive for users to switch from the original system they know and use currently to the new cloned system?

"switch"? You have to think what the flood of newcomers think. They don't have any stake in the original PTS and are in a disadvantaged position to get any. From the PTS clone/forks they have opportunity to become early miners/adopters. There will be clones of PTS if PTS is successful, just like alt-coins vs. BTC.

I am trying to get my head around protoshares.

Suppose PTS proves to be successful and DACs are paying shares to PTS owners. PTS price skyrockets. Many people wish they could mined some PTS but they can't because the door is closed two years after the first PTS had been mined. Then what if someone starts another copy of PTS, or a PTS fork that has different parameters -- for example with a better set initial difficulty so that the early miners wouldn't get most of the coins within a few weeks, or with more (or less for that matter) total coins, etc. ? Because the success of the orginal PTS there is no shortage of people the new PTS can attract.  Even many of the owners of the original PTS would want to get a piece of the new PTS because they understand the value of PTS and this time they can be an early miner! Since the marketing for PTS has already been done, new DACs won't have to use the original PTS to benefit from it -- the network effect and all, would they?

Actually if PTS is really as successful as its current believers believe, I can imagine there will be as many PTS variants as there are altcoins today. Will the return of current PTS be greatly watered down then?

If this question has been asked please point me to the answer.