That's the point though, as sad as it is, it's not "there" if it's not drop-in, given the stranglehold MRI has on the Ruby world.

You can run most applications with JRuby but to my knowledge, it always involves fiddling and tweaking.

For a Rails application, using the jdbc adapter or Puma/Trinidad instead of the regular servers for example. I am not saying this is a proper justification but it does prevent people from testing it further.

Also, I am still looking for actual and undeniable proof that using JRuby instead of MRI does actually bring better scaling/performance/memory management/tooling. So far, the examples I have seen are conclusive, but still limited to very particular use cases. That Heroku now officially supports JRuby is a big plus I would say.