If you've been around a while, you're likely going to have acquired favored virtual instruments and plugins. For these folks, the bundled additions aren't as big a "draw/enticement". For the beginner... or those without preferred solutions, the bundled additions are incredible value.
Very similar to the Komplete bundles. If you don't have any 3rd party virtual instruments/libraries/plugins, it's hard to beat the value of Komplete 9/Ultimate. If you already own Komplete 8, Komplete 9 is "much less exciting"... but still worth the investment.
I wouldn't base the decision to upgrade to X3 on "additional content"... but rather the improvements/enhancements to Sonar itself.
VST3, ARA, Colors, and Comping are all solid improvements that make hour-to-hour use of Sonar a better experience. If those things are useful to you, then the $149 upgrade is easily justified.
The additional content bundled with X3 certainly isn't a step backwards compared to earlier releases.
You're getting full versions of instruments/plugins.
Some are tied specifically to Sonar (Blue Tubes). This is due to license issues. If all versions were unrestricted, the cost would be significantly higher.