It's hard for me to fully-understand your thinking from just the comment above.  Maybe we can set a time to chat?  Here's my thoughts in no particular order, on why it would be good to not make active support an install dependency

1. Even though rbp is designed to run against Rails, 
  1. It doesn't actually depend on anything in Rails
  2. It actually can be useful when run against non-rails code
2. As a command-line app, there's no need to have Rails installed, even though it's a good assumption that it's available
3. Ditto when used as library
4. If you can improve the portability of your library code by assuming it's already installed and allowing for a fallback library when it is not, isn't that a good thing?
5. ActiveSupport is installed for its core extensions in many places where it really is not necessary.  I really wish it were not so closely tied to Rails and could be installed more piecemeal.  I would be willing to do that extraction if dhh would bless it, but it's not very likely.

I like the idea of removing the runtime dependency and not adding a new one.