I carry a little portable scale and always pack to or under the limit, but as recounted above, sometimes it goes over and I think some discretion (which is not auto permission) is just sensible.
As carry-on rules also cover dimensions, I'd ask all three of you where or not there should be discretion exercised to a mum travelling alone with an infant in arms and carry bags of nappies, food, pillow, toys etc plus a carry on. We've all seen it - very obviously over the dimension limits. Should she just be told to leave the bag of nappies behind, or the toys, or what? OR should the FA exercise a bit of kindness and discretion and allow mum to carry on?
Saying there should be NO discretion - ever - rules are rules, too bad, so sad, no excuse, no argument, no reason is just, well, silly and can be - heartless. Hope none of you are ever arrive at the gate 1 minute after boarding time. "But..but..but people have just gone through the gate lounge door and are still in the airbridge!". Sorry, sir/madam. Rules are rules. No excuses. See the sign says gate closed.