REPL requires [binding_of_caller](https://github.com/banister/binding_of_caller) anyway, which i doubt rails would include any time soon ;)
Re:Better Rails errors
by steveklabnik on Monday December 10, 2012 @01:51PM
(#4900809)
That's fair. :)
One of the things Rails 4 already features is better error pages, though no REPL.