Most glues are stronger than the wood, but if you are going to change
directions of the grain when gluing, such as long grain to side grain, the
natural expansion and contraction of the wood will eventually break the
bond. Also consider that table legs can be subject to loads that quite
possibly could be greater than the woods natural strength. The wood may
break itself. Using screws can add strength as the screw uses the strength
of the thickness of the board vs. the surface of the board strength.