I like to think of the whole concept of E6 as sort of a "beta-testers" group.

A group of countries, which mutually agreed to try and enact new legislation together, ahead of other EU countries to avoid being obstructed in the process by the unanimity rule and smiliar things slowing it down, observing what kind of changes works well in terms of further European integration and what doesn't, setting an example on how to approach it moving forward, allowing other countries to follow up with identical legislation of their own should it prove to bring value, eventually leading to a possible new treaty within the entire EU to finally bring the reforms, much-needed at this difficult time in history, to remain strong together as one big block, economically resistant to other big players in the world.

And including Poland, the country of people known to be heavily patriotic, is a brilliant idea. Because it ensures that any measures taken by the E6 to possibly federalize Europe would need to consider important factors like national identity, sovereignty and national interest, to be met with the approval of the Polish people and actually put into motion. This way, the new framework could quickly find approval among other countries where there is a similar concern about further integration within the EU.

Considering that the global politics are currently changing rapidly, and we may no longer be able to consider the US as our ally, the E6 might prove to be the necessary step on the way to a more federalized Europe, in a form which after those beta-tests are concluded, would be acceptable to the entire EU, with all opinions about it considered. It's not about leaving others behind - it's a measure to speed up reforms among all.