> a single service having multiple data layers for different things isn't really a big operational or organizational problem.

Your statement is true, and I’d like to consider the product/feature side as well. Enterprise SAAS customers expect their Single Source of Truth to be, well, a single source. They want reports across all of the data they have put into a product. A feature that should be a couple of JOINs can quickly become pretty dicey when data is sliced across multiple data stores.