If you have Gmail you have Google which means you have everything of Google including Google+ just don't put your real names look at the YouTube comments nobody puts their real name. If you say you don't have a Google+ account but you have a Gmail then you actually created a Google+
If a person wishes to stay anonymous while using any type of social network then by all means let them. I think that Google may be trying to hard to invade peoples privacy because they want to label its users on what they post on Youtube and that is just being nosy.
From an engineering point of view, it makes sense to integrate Google+ on Youtube, as it has already implemented many of the features, such as posting, threaded comments, sharing, etc. Therefore, in order to reduce development and update costs/time, they removed the old commenting system and did this.
HOWEVER, it is extremely inconvenient to have to make another account for a service that one does not remotely want whatsoever. I don't want any private data, like my name, where do I live, WHO ARE MY FRIENDS... To be made public, nor I want to be the star of Google ads.
Maybe they did this to minimize development costs... Maybe they did this to boost the number of users of Google+ and be able to compete with Facebook/earn a much higher ad revenue at our expenses. Who knows?
Well, I won't be getting Google+.