Drop the ehcache dependency. Caching is a separate concern.

I don't think Hibernate and Hibernate4 should depend on ehcache at all.

It seems to me that caching is a separate concern that should be addressed by a separate plugin (namely, the cache-ehcache plugin).

Setting up Hibernate caching with ehcache properly isn't as simple as how it's currently done in the Hibernate/Hibernate4, too:
cache.provider_class='net.sf.ehcache.hibernate.EhCacheProvider'
That isn't right because with that setup, there's a separate ehcache instance set up just for Hibernate, and it is only configurable using the default class path ehcache.xml (which falls back to ehcache-failsafe.xml). It doesn't get shutdown safely either (whereas the cache-ehcache plugin does safely shut it down) which causes problems when running distributed, or with a disk backing store.

I think the correct way to configure Hibernate with ehcache is as documented in the cache-ehcache plugin at https://grails-plugins.github.io/grails-cache-ehcache/guide/usage.html :
   cache.region.factory_class = 'grails.plugin.cache.ehcache.hibernate.BeanEhcacheRegionFactory' // For Hibernate before 4.0
   cache.region.factory_class = 'grails.plugin.cache.ehcache.hibernate.BeanEhcacheRegionFactory4' // For Hibernate before 4.0 and higher

You can see BeanEhcacheRegionFactory at https://github.com/grails-plugins/grails-cache-ehcache/blob/master/src/java/grails/plugin/cache/ehcache/hibernate/BeanEhcacheRegionFactory.java

In short, I propose the ehcache dependency be dropped from both plugins, and people use cache-ehcache if they want to use ehcache.