True, while it's dramatically serendipitous that an Enterprise is always the only ship that's available to answer the call, it is also apparently quite the effective strategy. An Enterprise at the scene never fails to save the day.
Makes you wonder why Starfleet doesn't name all their ships Enterprise.
Part of the Enterprise mystique is that the vessel is crewed by Starfleet's finest. Naming every ship Enterprise would only tarnish the vessel's reputation.
--Sran