The key issue with helicopter hydraulic systems with the size of the machine. Small helicopters such as R44, B206, EC120 have control loads that are light engough that should the hydraulic system failure the pilot is physically stong enough to move the controls manually. i.e. Manual Reversion. In bigger machines such as the Squirrel/AStar the loads become too large and the pilot simply wouldn't be able to move the controls. In this case you need a dual fail-safe hydraulic system.
In order to achieve the necessary levels of redundancy in the system accumulators are used to store hydraulic fluid under pressure, to provide a short period of control capability as you point out. As I say this is necessary in big birds as without hydraulic power you can't fly. With little helicopters you can manage without.....though it's not very nice.
Therefore in small helicopters accumulators are seens as needless weight and are not used.
Hope this helps
CRAN
