Well the extra money from the subscription wouldn't benefit Spotify as it would take money away from Spotify or it would increase the cost to customers.
A Spotify Application for the PlayStation would be a brilliant idea, a free download from the PS Store and tada! Unfortunately it isn't upto Sony to decide...
I see your point on the subscriptions but sony could take a cut from people who sign up via the ps3 rather then those who already have an account.
That would raise the cost of the subscription to the customer in order for Spotify to earn the same amount of money and give Sony a cut. Plus anybody with common sense would register using a Personal Computer first before downloading on the PS3.