Test Classes to Cover Multiple Triggers/Classes, or 1-to-1?

Background: My org has reached almost 750 unique un-managed classes and triggers. Until now, we had been adhering to the 1:1 test class to class school of thought. Now, we are at the point where we end up writing almost identical test classes just to cover 2, or 3, or 4 separate apex classes. We have a large number of validation rules that have to be met, therefore we often need to write ~40 lines to insert a single object.