On my own experience I use groovy not only for grails :
- virtually all java classes can be replaced by groovy classes. Groovy is far more concise. eg. I use groovy for spring/backed bean.
- SwingBuilder
- gpars
The Not-Invented-Here mentality has always been prevalent in the VMWare-funded "Gr8te" ecosystem. If someone independently builds something using Groovy or Grails, such as Alex Tkachman, Roshan Dawrani, et al building Groovy++, VMWare make excuses to reject it, then build their own version, Groovy 2's static compilation, probably copying plenty of code out of Groovy++ along the way.
This mentality results from VMWare wanting to control as much of Grails's constituent technologies as possible. The reason behind Gradle becoming the build tool for Groovy/Grails, Spring, and Hibernate was for Gradleware to pitch their control of Hibernate's build, knowing they'll be targeted for a buyout by VMWare. Rocher's probably hard-balling the Gradleware execs right now, trying to screw them for as much as possible.
There are plenty other reasons to adopt Groovy, other than Grails
Your other statement are opinion, not fact.