I have toured on three tandems, two steel with a diagonal, and one titanium without a diagonal. With the current tubing available, diagonals are not necessary. The only real advantage of a diagonal is for water bottle placement. I am able to stand out of the saddle without a wobble on all of those tandems whether we have front and rear panniers or just rear panniers. I am a little wobbly at the beginning of a trip when first standing, but I quickly adjust to the panniers and then have no problem.