Well if he wants to drive next year I think he should. Despite the widespread (and kinda correct) opinion that this comeback has been a disaster, I think he is still one of the top 10 drivers in this very competitive F1 field.
In all but the official championship points table, Michael has for the most part had the measure of Nico this season. Michael may have struggled a little in China, Hungary and Singapore, but I don't see anything like what Rosberg managed at Sepang, Silverstone or Spa (i.e. performances that rival Massa's, in the midfield and outta the points while Michael's fighting around the sharp end)
I'm sure that in absolute terms, Michael can easily beat drivers like Kobayashi. So if he wants to drive, there should be a good place available for him.