I ask "Why?" a lot when I am using software written by other people. This is largely due to my hackerish nature and experience with OSS.
I get worried when the answers to that "Why?" do not make a lot of sense, especially when it is some thing like a framework that I invest a lot of trust into. Rails failed to have a good answer a number of times when I dug into it. Most of the time it was a "that's the way it is" type of scenario.
Merb never does this to you. Merb always answers the "Why?" with things like "Because you chose it to be that way" or "Because there is an HTTP spec to back that up." It all makes sense.
Other reasons that are related to the above that will keep me coming back: it's very fast and very flexible.
Its agnosticism is attractive in certain situations. Rails is rather tied to ActiveRecord (Merb uses DataMapper by default, but you can choose others) and Prototype (though Rails has an unofficial jQuery plugin)). Merb was designed ground up to be agnostic on database, view and JavaScript choices.
Merb is "lighter", faster in many situations, and has a different "flavor" generally, that you'll either love or feel indifferent to. It was designed to be thread safe almost from day one so this element is well tested, whereas Rails is only just getting into that area with Rails 2.2.
Disclaimer: I like to support Merb, but I'm still a Rails user generally.
