Performance and improved introspection. Once they're first-class they get included in that class's list of methods, which is helpful sometimes. Mostly I just li

Performance and improved introspection. Once they're first-class they get included in that class's list of methods, which is helpful sometimes. Mostly I just like to avoid m_m when I can. AR is actually an okay example - it's as good an illustration as any of a place that employs these kinds of techniques, and the source of a lot of complexity in Rails.