Atmel suggests a RC reset circuit for the chip because they want to sell to toymakers.
If you are making toys where switch off twice once in a while and brownout malfunctions "does not matter", go ahead with the RC.
However if an occasional malfunction of your project matter at all, use a supervisor

I fail to understand why, after a host of knowledgeable participants (such as Kai, Ijaz and Andy), in this forum has posted "avoid RC reset" the darn thing keep popping up.

Erik

Kai, Ijaz and Andy:
I did a quick search to find a few names, not trying to "get you in trouble".