You need to understand some of the physics involved in the structure. Think cardboard box open on both ends, so you can see through like a tunnel. Kinda collapsable, huh? Close one end...still not very good. Close the other end ... much better.
Simply put, rectangles collapse, unless reinforced with either panels, stretchers or triangular braces in the corners. The mortise and tenon acts like a small brace and the wider the apron the greater the strength.
Adding an additional stretcher at the bottom creates an additional brace, however it's still a rectangle from the side so it's not a very rigid structure, but it's way better than none at all.
Triangles will not collapse unless the joint fails. A rectangle with a diagonal brace will not collapse unless the diagonal comes off or the joint fails. Farmers and harvest tables and benches often have a center stretcher or rail which then connects to the legs by means of a cross brace.
Benches and tables are subject to horizontal side forces when bumped into or moved/dragged with loads on them, so the bracing is necessary to prevent racking and joint failure.
Here's some examples of tables.
Some have no stretchers, just aprons, others have stretchers: http://images.search.yahoo.com/searc...=harvest+table
Here's some simple benches:
I think this one is particularly beautiful and has some hefty mortise and tenons! http://images.search.yahoo.com/image...mb=AyhczztaeH5