Perhaps the advantage of #DEFINE is that you don't need to explain why you're storing a constant in something called a "variable".
FWIW, if you do use variables then you need to pay attention to the m. performance issues, especially if you're doing calculations inside tight loops. Without the m. prefix, VFP will have to check for a field of the same name in the selected table every time the variable is referenced, which does not happen with a #DEFINE.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
