Java Java Data Structures Efficiency! Custom Serialization

Why not use a lightweight database when prototyping the MVP?

Since the ultimate goal is to store/retrieve song and artist data in a database, why spend time coding a purely in memory data access paradigm with Arrays and Maps? Isn't that wasteful since it will be discarded in a production version? Or is the cost of developing a temporary data access scheme based on in memory constructs less than creating a rudimentary database when rapidly prototyping a solution?