Slipstream's schedule is posted. If this is an accurate schedule (which I assume they have invites to most of these races), why would a team pay the money to join the Pro Tour?
It looks like they are going to miss Amstel Gold, Romandie, and Tour of Switzerland, but that is about it. They will be in all the other big races the first half of the season including the Giro (and maybe the Tour). The will be in Milano-Sanremo, Paris-Roubaix, LBL, Paris-Nice, etc.
If I were running a team, I would consider saving my Pro Tour dues and the other obligations that go with it and staying as a pro continental team.
If Slipstream is able to race almost all of the high profile races without joining the Pro Tour, I wonder if others will follow the same format.