1)When do you have to use a reset/supervisory circuit? 2)Is it always necessary?
1) when you do not want to be in doubt (is this due to a failing reset or is it something else). When you have any kind of non-volatile memory a supervisor with brown-out detect is mandatory. Preferably a two stage.

2) no, not necessary if you can live with the doubt as to why did this fail.

I remember a product designed by someone else that did occasionally fail on power-up and the problem was "accepted" as a side effect of the RC reset and "starting again is no big deal". When someone finally decided that it was a big deal and a supervisor was installed the problem still existed and was finally fixed. The old RC boards were upgraded with the new software and while they still occasionally refused to start the frequency of occurance dropped markedly.

So, keep your RC if you can live with not knowing why and install a supervisor if you want to be sure.

Erik