The RX100 is not available yet but you are on to something. Allow me to replace the Sony RX100 with the Canon G1 X in your question and answer that instead :)

A large sensor camera gives you high quality images, particularly at high ISO and shallow depth-of-field compared to typical compact models. This is true for fixed lens cameras and interchangeable ones like the E-PL2.

Technology wise there is no reason why there would be any difference between a fixed lens camera and one with interchangeable lenses used with exactly one lenses. However, practical differences are more significant and vary between models:

• The majority of lenses have mechanical zooms. This makes zooming silent, quick and most importantly much more precise.
• Mirrorless cameras can have built-in or optional EVFs which is found to be a more pleasant way of shooting than having the camera at arms length. Not all find this but using the camera at eye-level is more stable.
• Some mirrorless cameras are weather-sealed. No large-sensor compact is.
• If the G1 X is an indication, the latest crop of mirrorless models focus much much faster.

On the other hand, cameras like the G1 X have a tremendous appeal for people who want quality images and simplicity. No changing of lenses, no sensor dust issue, etc. You give up flexibility obviously but only if you are going to use it. Many DSLR owners and mirrorless owners never buy a second lens. For those people, a fixed lens camera with very high image quality will certainly do.