Don,
The tongue-in-groove joint is fully adjustable, this lets one use many different frame angles, including raising or lowering the bottom bracket and/or having many different seat stay angles or wheelbases without difficulty. With a lug, you'd have serious problems between 20" frames and 26" frames on the upper socket; it would have problems because the stay angles change for different frame sizes (since the wheelbase stays roughly constant), requiring extra brass and filing to fill the crack(s) that would be created.
- Don Gillies
San Diego, CA