The TPP covers enough of the current Global Economy (some of South America, Some of Asia, North America) so that it serves as a blueprint for future Trade Treaties between the current signatories and future signatories. It's important to get the deal crafted in such a way that anyone who wants to join in the future has to basically take on the same compromises and advantages. And make no mistake; other nations will want to join in the future.
If you don't get this deal done, you run the risk that future trade deals compromise your position further than maybe your country feels is reasonable. One thing that is almost certain is that the current signatories are looking at a weakening economic position in the future; by signing and crafting now, they negotiate from a position of strength that probably will not exist to the same extent in the future.
It might not be ideal for all the current signatories, but at least now we know where the advantages and disadvantages will lie, and current treaty nations can adjust accordingly so that they are in a position to take advantage in the future, with rules the future signatories will have to agree to.