Thanks for the replies, all! Sorry if I can't reply to most of you as I only have so much time at work.
Certainly, there are obvious benefits to maxing out your primary stat: This includes better attack rolls, spell save DCs, damage bonuses, etc etc.
I was just wondering about the psychological reasons for wanting to max out. I noticed that I'm disconnected from the majority of my gaming group, you see, in this regard. For me, if my attack stat is 16, at maximum proficiency that puts my attack roll at +9 (assuming I never find a magic weapon) and my spell save DC will be at 18. In a world where ACs range from 9 for an ogre to 18 for a humanoid in chainmail and shield,* my attack roll seems good enough. At what point do you get so good at what you do that getting even better becomes redundant? For me that's at 16, possibly 18. After that I'd want to look into my other stats, or take interesting feats once available.
But then, take note that I'm usually the party bard, so this kind of jack-of-all-trades mentality may just be hardcoded into my brain.
Anyway, I'll actually be the DM of our group for 5E. So primarily I wanted to understand a mentality that isn't natural to me as it will affect how I handle my encounters.
Which brings me to:
Primary stat for many classes isn't for primarily for skill rolls for most classes, it's primarily for attack rolls.
Missing or hitting is often life-or-death. This is only MORE true at lower levels when your HP pool is more shallow.
For casters it can be less of thing, because in many cases your stat isn't a big part of what your spells do (in 5E). For melees, though? Kinda nuts.
For Rogues, who have both most of their skills AND all their combat abilities coming off DEX? Well, not maxing it would be a pretty extreme decision.
Of course if your DM hates TPKs or Challenging/Hard encounters, it's much less of an issue (esp. if he has a lot of skill-based stuff, then you may benefit from spread stats around).
I feel like TPKs are still possible even if DM sticks to medium DCs. Even at the highest levels, I don't think everyone and anyone the party fights will have ACs of 20+ or attack DCs of 20+. But their collective damage can still hurt pretty bad even if their AC/DC are 15-ish. I guess we'll find out once higher-level monsters are revealed.
I see the life-or-death thing with missing at low levels. This is probably right, which is an incentive to get to 18 as soon as possible. By the time you have the chance to get to 20, though, you may not be so low-level anymore so you can probably survive missing once or twice.
As for the rogue who didn't max out his dex, wouldn't he, at 4th tier, have a dex skill of +9 (+15 if he has expertise?) He gets +11/+17(exp) if he maxed out, and I don't think it's necessarily extreme. Again this is probably my bard-brain talking.
