I can name one exception using an example you gave. You said that it is justifiable in some cases to use a 20 in your attack stat. As a weapon user, that gives you a +1 over the 18. If you take a +3 proficiency weapon, that gives you another +1 over the +2 proficiency weapons. That makes expertise not needed on the 20 primary character with the +3 because he's has the same hit probability as the 18 with the +2.
No. If a character is justifying
four dump stats in order to be really good at hitting, he's quite likely to want to be ... wait for it ...
really good at hitting. Expertise might be the second or third feat he takes, because if he's willing to pay for a starting 20, he's likely to be willing to pay a feat to get that extra +1.
Otherwise, he's probably taken a fool's bargain: losing his tertiary ability bonus will usually cost him more than one feat could buy back.
I'm open to well-constructed counterexamples. Good luck finding one.
there's no mathematically better option by the time you're looking at your 8th feat (14th) and expecting it to jump to +2 at 15th.
There's seldom a mathematically better option by
fourth level. It's just too obvious for anyone to ignore when it becomes +2.
At +1 to attacks, it's better than every other published feat.
At +2 to attacks, it's
obviously better than every other published feat.
At +3 to attacks, it's not even worth talking about. If you don't take it, you're doing so out of nothing more than autonasalectomic spite.
Cheers, -- N