See for yourself:
 
https://www.youtube.com/watch?v=CSUWxrA7znc
 
 

Very nice Andrew.  Such a simple thing, but I want it!

Another way to do something that's already pretty easy. Meh, sorry.

Another way of looking at it, is that this is a "making things simple and obvious for new users" feature. I very much welcome stuff like that, as the more new users Sonar can attract and retain, the better for me in the end, as the company is a stronger position.
 
I've long thought that the remaining open battleground between DAWs is mostly around with ease-of-use and shallowness of learning curve.

ampfixer
Another way to do something that's already pretty easy. Meh, sorry.

Right clip menu, keyboard shortcut, control surface binding... But on that place I currently see time display, which is definitively more useful that this "+" button. Is it going to be removed?

I like to explore soft-synths and libraries. Currently I'm working on several projects where each has just one synth with a particular library. This new Add Track feature appears to be great time saver. It might not be as fast as using templates, but seems very versatile and easy to use.

I like that.

Soundwise
I like to explore soft-synths and libraries. Currently I'm working on several projects where each has just one synth with a particular library. This new Add Track feature appears to be great time saver. It might not be as fast as using templates, but seems very versatile and easy to use.

There is already "+" button in the rack to do the same, with more properties in "Insert Soft Synth Options". And I see one difference in that video, which potentially can render new button almost useless: there is no "Enable MIDI Output" option and in current version it is enabled by default (the reason for many posts from people which use "Omni" MIDI input).

azslow3
But on that place I currently see time display, which is definitively more useful that this "+" button. Is it going to be removed?

I don't think I've ever looked at that time display.  In fact I had to bring up my current project to see what you were talking about.  I use the Control Bar's clock which I have positioned dead center at eye level.
 
I like this new function.  I've just recently begun using the "+" button in the Synth Rack instead of double clicking in Plugins/Instruments, and this similar button will fit right in.  It's certainly more obvious to new users, as has been mentioned.
 

Nice I like this.
Hope I can use a keystroke to open it (I probably will if I look hard enough, or if someone tells me)
Seems easier to get a hold of for new users as well.
All the best.