For over two years the new Gmail users were required to have a profile on Google Plus, Google made a move in 2012 and was widely criticized by users.
This requirement made the numbers Google Plus users aumentasen considerably, although not all were active at the end, so it did not reflect the popularity of the social network, which has probably been the cause for the existence of some independence between the two platforms.
They comment on Google from wordstream.com few days ago that there is no such obligation. A user can decide at the time of registration if you want to have google Plus account or not allowing, if not, then the record is made.
For an account on google plus enough to publish some content once in the network, creating the registry automatically.
I doubt that it is another move to prepare closing Google Plus, and I personally find there a lot more interaction than other networks, but it is clear that the philosophy of this platform will change a lot during the coming months.