On page 122 of Gratzer's book More Math into LaTeX, he says that
The tabbing environment has to be typeset with typewriter style font.
And on page 123 of that book, he says that
You do not really have to use typewriter style font—just beware of the pitfalls.
My question is what can happen if I do not use the typewriter style font to typeset the tabbing environment? What does the pitfalls mean?