Not at all, in fact the enemies basically wind up being red herrings.
They drop trivial amounts of souls which are used to buy levels, but souls are also gained by beating bosses and from using items that also grant souls.
I've experimented a lot with this. If you just fight bosses and use boss soul items and other easy to reach soul items, you will end each game roughly 5 levels below someone who is more meticulous and clears each zone of enemies without grinding them repeatedly.
At the end of the game, those 5 levels will be a trivial difference.
They drop trivial amounts of souls which are used to buy levels, but souls are also gained by beating bosses and from using items that also grant souls.
I've experimented a lot with this. If you just fight bosses and use boss soul items and other easy to reach soul items, you will end each game roughly 5 levels below someone who is more meticulous and clears each zone of enemies without grinding them repeatedly.
At the end of the game, those 5 levels will be a trivial difference.