Sparta said:It's sometimes better (IMHO) to sacrifice a siege weapon or two than to risk several axes, maces, or the like in attempting to take the city.
I'd go so far as to say that this is a fact, rather than simply your humble opinion.

A unit that can knock off a good deal of damage from all the units in a square is clearly good for taking cities. All you need is enough siege units to bring the defenders down to 1-2 strength, and then 1 ground unit for each defender, to mop up. (This is the minimum. You will probably want more to defend the city after you take it.)
In summary, collateral damage is the key to taking cities.