There was no imperative to attack or destroy that SoD, unless it was not on a good square/receiving no defensive bonus, or if it was in a city. How often could you get a SoD onto a good square where the enemy was forced to destroy it to move forward ?? Almost never !!
Sorry, if this sounds harsh, but I don't find better phrases.
You seem to have missed to really think about Civ4's combat system.
Your enemy approaches you with an attacking SoD (aSoD) of say 50 units. Some of them are things like trebs and city raider promoted units.
Now, if you can move a defensive SoD of your own (dSoD) into their way, these trebs and CR units are rendered at least much less useful.
And now imagine that you place your dSoD into a forest next to their way (alternatively, behind a river, on a hill, on a forested hill, on a forested hill behind a river). Since the forest gives you 50% defense bonus (not taking any units with Woodsman promotion into account!), your dSoD can be much smaller than the aSoD.
You may attack the aSoD with some good attacking units (based on territory) to inflict some attrition on the attacker and still chances are good that you may hold your position against a counter attack.
And even, if that counter attack would be successful, then you would still have bought time for your cities to produce new units and the aSoD would be much smaller and wounded afterwards, therefore imposing less threat to your cities.
Since your first dSoD could be smaller, you could generate 2 dSoDs, to block the enemy path of approach even better. So, if the aSoD would not care about dSoD#1, it's path could still be blocked by dSoD#2, allowing dSoD#1 to redeploy into a new battle position.
Just waiting for the enemy by putting both dSoDs into a city would be harmful, as the attacker then could:
a) pillage your territory,
b) would at least block you from accessing at least one field of the city's fat cross
c) make use of all city attack modifiers
So, defend your cities as far outside as ever possible. In the long run this will save you units and time to produce even more units.