Very desirable, but linear scalability is often a constraint of the business logic not a fundamental property of the database. For instance, if you are Twitter

Very desirable, but linear scalability is often a constraint of the business logic not a fundamental property of the database. For instance, if you are Twitter where everything is an append-only insert then achieving linear scalability ought be easier than if you are Google Docs where every update effects many other dependent records. Most benchmarks (and this isn't a bad thing) assume the former.