Reality: The very steps that bring you to a certain level of mastery are the ones that will help to maintain that level of mastery. Most people assume that practicing hands separately is a temporary step towards the goal of playing hands together. But let’s examine what professionals do. They will continue to practice hands separately on a piece they’ve played for 25 years! This is because only when you practice hands separately can you really focus on certain nuances that would be obscured if you were spreading your focus between two hands.
The better hands are mastered separately, the more reliable and proficient it will be when you then bring hands back together. So, please think of practicing hands separately not only as an initial phase of learning a passage, but also as a way to MAINTAIN and also to further improve and polish a section of a piece you’re working on.
-piano teacher Howard Richman.