actually, that's my tactic when besieging a tough settlement...I make it a point to punch holes on the defenses as many as i can...continuous bombarding does the trick(at least for me) and a heavy all-out assault follows...is it a lousy strategy?
That's the strategy I use as well. But once you have knocked several holes in the wall, instead of rushing them, attempt to knock down the gate (line up your army like you will rush only the gate) and take what was left of the walls? I would have let the AI do this if it wasn't for his horse archers!