Mortise and tenon joinery is great insurance against glue joint failure due to poor gluing technique or differential expansion and contraction between the sides of a box and a back panel that has been glued into a box with glue that is inflexible when it dries.  Mortise and tenon joinery can also help resist glue joint failure due to wood that may warp due to the expansion and contraction of wood that occurs when the moisture content of the wood changes.  Mortise and tenon joinery can also keep the box from breaking if it is accidentally dropped onto a hard surface.  I recently looked carefully at a group of Sailors Valentines on display at a gallery.  Every Sailors Valentine on display had at least one of the eight joints in the box that was at least partially separated.  That's the sign of glue joint failure.  

The strength of a simple butt joint depends entirely on the strength of the glue and the use of good gluing technique.  I have purchased several inexpensive Sailors Valentine boxes sold on eBay and other venues over the years and tested their strength by dropping them on my shop floor.  Every one of them broke in at least one of their joints on the first drop.  The same would be true for the boxes I make with simple butt joints.  They may survive a fall onto a plush carpet, but falling onto a hard surface is another story.

I visited a local frame shop to test the strength of simple butt joints fortified with V-nails. V-nails are commonly used by framers to strengthen the joints in rectangular picture frames. We tested three octagonal frames that I had made for the purpose of the test. One box was given a single V-nail in each joint; one box was given two V-nails in each joint; and the third box was given three V-nails in each joint. Each box was dropped onto the concrete floor from a height of about five feet. At least one joint in each of the three boxes broke the first time the box was dropped. V-nails might keep joints from separating due to changes in humidity; but, they do not provide the strength of mortise and tenon joinery from impact loads.

In contrast, the much superior strength of a mortise and tenon joint is due to the added strength of the wood tenon which spans across the glue line between two adjacent box sides.  For a mortise and tenon joint to break, not only does the glue line have to break, but the tenon has to break.  Considering that the grain in the tenon runs perpendicular to the glue joint, it is very, very unlikely that the tenon would break unless the box were being deliberately abused.  I tested the strength of my mortise and tenon joints by dropping four different boxes three times each from a height of about five feet so that a corner of the box hit the concrete floor of my workshop.  Not once did a joint break during these tests.