I polled my session attendee's at Hotsos 2014 ( Three approaches to shared pool monitoring ) and asked them who was using automatic stuff versus who was doing it all manually?
My guess is that there were about 100 people in the room and it looked like automatic "won" but not by much. Maybe 55 percent automatic and 45 manual ... so there are still a whole bunch of people nervous about using that functionality.
My presentation was updated ( after delivering it ) to add in some additional doc id's and warnings ( both pro and con ) about manual memory settings.
Specifically worth noting is the un obvious fact that EVEN IF you have disabled oracle from doing SGA resizing operations automatically ... well it may do them anyhow. Makes me a little grumpy eh?
So you can set SGA_TARGET to zero ( and/or MEMORY_TARGET to zero ) and still have oracle make decisions for you? It turns out that after some point you have to set yet another hidden underscore parameter to tell the oracle software "yeah thanks but I really really mean it" ...
Oracle doc id 1269139.1 ... a pretty good writeup here ... really turn off ammasmm