This is a great question.

It took C# (formerly a vb programmer here) to teach me that collections work quite efficiently using delegates and predicates and, bearing this concept in mind, it makes far more sense (to me) that an "exists" functionality will provide a faster and more efficient query than one searching the index for a value (IN statement). In this respect the query simply records the rows that are true... or well, at least, not being fully aware of the inner workings, it makes sense to me that counting and returning items that are true is more efficient than searching for them by name and returning them.