A little while ago, I built a test platform to test different point defense builds. The platform fired 14 dumbfire missiles every second straight out, bracketing the area I placed the point defense systems I was testing. I was able to get a pretty good visual guestimate of the effectiveness of various point defense systems. (Waffleboard cannons don't improve PD at all.) Later after completing my latest battleminer that had eight point defence turrets on it's front (four on the rear), on a whim I parked the ship in front of that platform so the dumbfire missiles would fire straight at it.
14 missiles per second... I sat there mesmerized for ten minutes as not a single missile hit the ship.
Now I must admit these were no ordinary point defense turrets. I had spent a lot of time perfecting a PD turret that is extremely effective. And no, I don't particularly feel like divulging my secrets, again. Suffice to say, that there 'can' be more effective designs. That said there is nothing so wrong with regular point defense turrets that having a few more of them won't fix.
When building an offensive strike missile system, I will indeed endeavor to saturate PD defenses.
My Corsair pirate for instance has nine missiles that it will launch as a single wave, but only one of them does significant damage, the rest are decoys. My standard ship strike plan is to strike with a wave of over 100 missiles. Most of those will be two module missiles designed to be decoys for the 'real' missiles hidden among them. Not even my point defense can shoot down 100 missiles coming in as a single wave. Many of the big warheads will get through.