I believe that Sonar provides the extra plugins in partnership with the other companies to provide nice extras - to sweeten the deal for those interested in getting such extras, and it also gives companies a nice way to introduce a bunch of folks to their product lines.
In doing this, Sonar is able to offer these extras without adding significant cost to Sonar. I have gone out and either upgraded some plugins or reviewed and sometimes purchased other plugins from companies that introduced me to their plugins through being included in some version of Sonar.
And, even if a given plugin is tied to work in Sonar only, or some other DAW only, other than having to build tracks using those plugins in the DAW they are tied to, one could always bounce those tracks for purposes of doing some editing or tracking in some other DAW, bringing the edited tracks back into Sonar to continue. It is by no means a perfect solution, but since they don't add much, if any, cost to the purchase price of a Sonar license, I am very happy to have whatever extras they can throw into a given version of Sonar I am contemplating purchasing.
My son and I also purchased FL Studio, and a bunch of the plugins in there only work in FL Studio - I would imagine this is true of most, if not all, of the major DAW products on the market. We both like some of the work flow in sequencing in FL Studio, and sometimes my son starts projects in FL Studio and then moves them to Sonar for the heavy lifting. Many people have more than one DAW product, and there is nothing wrong with that. All of the software is simply a collection of tools, and it simply gives folks additional options at creating projects.
I have an Intel CPU, but not an Intel motherboard, and other computer parts from a whole host of vendors, and nobody cares - all of the parts are just tools that together create the bigger picture - the whole PC. All of the various plugins in Sonar are sort of like that, and they all are part of the bigger picture - the software and hardware that is a robust audio engineering suite that is Sonar on a monster PC.
Bob Bone