Unsupported beam-to-beam connections apply large concentrated downward loads to a relatively small surface area along the beam. This type of load puts a lot of shear stress on the beam, which under certain conditions could lead to the beam snapping and a catastrophic failure.  Point loads distribute weight differently than when using joists that distribute the load evenly across the length of a beam.  Unsupported beam-to-beam connections are only acceptable when used in circumstances where the beam will only be carrying a very small load such as attaching a small stair landing. 

It is always preferable to position a structural post underneath any beam-to-beam intersection.  Never attach an unsupported beam-to-beam connection to the center third of a beam or to any section of a long beam. You may need to redesign the frame to avoid generating point loads. In most cases it is best to consult a professional engineer to evaluate the specific situation.