Now i was trying to understand what experienced divers would judge useful and what not completely necessary
They key to any type of ongoing education is making sure its in an area that interests you. Many divers do courses in new types of equipment or environments (drysuit, wreck, deep etc), whilst the emerging trend is to do courses which can help you to become more aware of the aquatic environment you're in. These might include the AWARE courses or the likes of Reef Check, Coral Watch etc. There is a real demographic split as to whether these are "useful" or not, but that will come down to whether you want to be taught about the evironment, or whether you prefer to pick up a resource and learn about it yourself. The right instructor can bring the subject to life, whilst one who is only going through the motions can make you want to end yours.....