Nate, there are two reasons I use the <input type="submit"> in that scenario:

  1. The pre-IE8 bug (along with general really-old browser support);
  2. In my experience working with teams, leaning on <input type="submit"> for a <form> and a <button> for a non-form use helps reinforce the purpose of the control with developers. This has had the effect of getting them used to making decisions about client-side scripting, error handling, styles, and so on as their brains are primed for a specific type of interaction belied by the element chosen. Of course, YMMV.