I've had next to no use for spies at all.
-- They're tough to insert. I've succeeded exactly once in my entire Civ3 career.
-- I've had very few occasions where stealing technology would be handy. Extorting a tech out of the enemy in exchange for peace is more reliable.
-- Never had a need to see enemy units, but this is due mostly to my style of play. If I'm fighting a defensive war, I can already see all the units in my territory. If I'm going offensive, I'm attacking ONE opponent, usually one city at a time (the no-roads rule when in enemy territory makes it hard to blitz multiple targets), and attacking with everything I have, regardless of how much force I really need.
-- Propaganda?? Bah. Enemy cities (which are usually high in corruption) aren't worth spending money on. They deserve my wrath, not my wallet!!!
Guess it's all just a matter of how you play.