I'm generally not a fan of subclassing core types.  @steveklabnik touched on this recently:

http://words.steveklabnik.com/beware-subclassing-ruby-core-classes

Even if we don't run into any of the specific problems mentioned there, it has enough gotchas that I reach for it only after deciding against other options (particularly composition) for whatever reasons.

Is it really needed here?  Can we use composition instead?