Besides the obvious advantage of choice between rendering engines (performance, stability, bugs, compatibility, development) this allows also the user to pick either one while not having to relearn a new interface nor does she have to unlearn the old interface hence allowing the user to go back on forth because this part is the same. If you ever use an alternative keyboard layout and switch to another (forced or otherwise) you know what I'm talking about. E.g. laptop -> desktop, qwerty -> dvorak, or language layouts. Or say IE at work, with a real browser at home.