I just wrote an answer to @ememovic who had posted the same question.

I said:

I've been wondering that, too. If you want the skeptic's point of view, check out Mr Walt's youtube video on Plexcoin - search "Plexcoin Sean Walters" on Youtube. He has a lot of good points about key information that is missing from Plexcoin's presentation, i.e., what makes it seem shady.

There's three possibilities as far as I see it:

It's a great new cryptocurrency with brilliantly original technology that they just haven't managed to publish a white paper on yet.
It's a scam and you're likely to use money you put in it.
It's a scam that if you get in early you'll make some money but get out fast!