Yes, because a rules system will always create more uniform results than a non-rules system.
A non-rules system.... So basically you are advocating no rules? Ok.... The thing about a 'no rules system' is its really easy to provide you one, but maybe you shouldn't get an opinion as to what the rules are since you aren't going to use them anyway.
Here's the alternative:
Old seamstress: Dex 14, Con 8, Str 8, all other stats 10.
Even a 70 year old feeble grandmother has a dexterity probably higher than any of my current players and is certainly on par with many a 40 year old nerd in overall health?
AC 10(she doesn't get a Dex bonus to AC because she's not a PC and doesn't have to follow their rules, but she's good with her hands and fine manipulation of items so she has a high dex).
In D&D sewing is a craft skill and as such has nothing to do with Dexterity. I suppose this is another rule you aren't following. But I really have to ask here, why she needs stats at all? What purpose are they really serving you, since they aren't markers for anything in particular? She has a dexterity score that normally increases initiative and AC, but apparently in your case it isn't. Meanwhile what it doesn't do is make her a better sewer. So what in the world is it for? Is it showing she can dodge fireballs? Which rules are you going to follow and if you aren't following the rules to get the outcome you want, why should the players trust you?
+0 to hit for 1d4 damage with sewing needle
1d4 damage with a sewing needle? A sewing needle??? That's a really big needle. You know, there is something to be said for verisimilitude at times. We're dealing with something about 2 size classes smaller than an ordinary dagger, dealing comparatively enormous damage (1 stab potentially kills your character).
Young seamstress: Dex 16, Con 14, Str 14, Int 12, Wis 10, Cha 12.
That's what, 34 point buy? Your young seamstress has better attributes than any of my PC's. Indeed, that's a better stat array than any NPC I've introduced into the game in 3 years of regular play. My BBEG isn't so completely well-rounded. Why the heck isn't she saving the world instead of the PC's? Stat arrays like that just scream DM's pet.
You've got no consistency at all and you seem to take pride in that. How in the world do you expect your PC's to get proactive and make plans if it isn't possible to predict how your world is going to behave?
Wait, why would I have to calculate the points spent building the character or set limit or demographic assumptions? I just needed a seamstress to act as an NPC in my game. I just came up with 2 of them off the top of my head just above. I didn't follow any rules at all. I just set what looked right for the character I was thinking of. It took about 5 seconds.
Which is about how long it took me to jot down my two examples. So yes, that was trivial too. As for why I don't like to arbitrarily assign stats, I've already answered that several times. I feel your examples just reinforce my points.
That was trivial? You've created a 70 year old seamstress who is not just old but extremely feeble and barely able to get up and move around.
Most 70 year old women aren't exactly spry, and you are the one that explicitly described her as 'frail'.
She's really bad with her hands and manual dexterity, precisely what she needs to sew.
No she isn't. According to the rules, Craft is an Intelligence based skill and so provided she has skill ranks in Craft (Sewing) and continues to have a reasonably sharp mind in her frail body, there is no reason why she can't be a fantastic sewer. As a 4th level human, she has access to 3 feats to boost her skills, 7 skill ranks, and probably a point or two of bonus from her intelligence. Depending on which sourcebooks you use she could easily get a +14 bonus to her Craft (Sewing) skill checks, and probably more. In mine she'd take Skill Focus, Skill Aptitude, and Craftsman for total bonus from feats of +7, granting her like a +16 bonus. Throw in some masterwork tools, and that hits +18. I've spent virtually all her resources on sewing. She's a narrowly focused character who excels the PC's in sewing, but in virtually nothing else. She's balanced within her universe, and there is no power creep, no stat inflation, no reason for the PC's to be jealous of the NPC's because the DM's characters can break the rules or receive benefits and coolness they never possibly could attain. No one can accuse me on the basis of the stats of impartiality, of favoring my plots and my characters over the PCs.
Her stats should all be close to 10 as she's fairly average
A ten is average only for a young adult, or at least the population as a whole. A 70 year old on average does not have a 10 strength and a 10 dexterity. A 70 year old woman who is as strong or nearly as strong as an average young adult male is not average.
and I'd like her to be able to carry large bundles of supplies back from the market.
Which implies she's not really 'frail'.
Meanwhile, you've had to look up and find the description of two feats: Dodge and Toughness...
No I didn't.
which the NPC will never use or care about since this NPC will never get into a battle and is just here to be a seamstress.
Then why does the seamstress need stats at all? I'm never going to bother with stats unless I think they are relevant. But again, I don't know what NPC's are 'here' for. The character could end up anything from a paramour to a foil, or an ally, or a ward to be rescued, or a friend whose murder I use to hook the PC's into plot thread. I might not have particular plans when the character is first introduced. It could be the character was originally just furnishing and color, and became important because the PC's focused on her and enjoyed the resulting spontaneous role play.
You had to roll or figure out the average of the hitdice to figure out her hitpoints
No I didn't.
you didn't list her AC so if I do have to use her in combat I don't know what it takes to hit her.
No, but it's pretty easy to figure out if I wanted to figure it out. I didn't jot down everything about her because it was an example I spent 5 seconds on.
If I wanted to figure it out I know have to look up the AC of Masterwork padded armor which I likely don't know off the top of my head.
Then you should be learning the rules then.
I then have to add the Dex bonus.
That's not hard.
When I use her in combat, I now how to look up Dodge again to figure out what it does.
Maybe you do. I don't.