If i was a non Mithra then yea it may be best to not bother adding DEX but if you add DEX to an already high amount of DEX it must help with crit hits?
I know i cannot reproduce that outside of Abyssea but DNC gets alot of +DEX on the AF3 and as Mithra with a high DEX pool and perhaps +5 DEX merits aswell... would that push up the crit hit ratio outside of Abyssea and make it more noticable?
Crits from DEX don't go up an infinite amount. For reference
50 DEX over a mob's AGI is supposed to cap crit rate (at least, that's what Greater Colibri parses indicated for 75). Inside Abyssea, with Razed Ruins especially, you're likely already hitting that cap. You get more by forcing it via direct addition from Atmas or gear stats.
Outside, since you can't readily be sitting at +100 or more DEX, it's not as readily done...but it may also depend on the target mob. Exact AGI isn't known for most mobs - mostly only tested for merit ones I think. Regardless, it still takes swapping DEX in to cap crit rate for TP for many mobs - Greater Colibri are some of the easier mobs to cap for (at least in terms of old merit mobs) and those need 117 DEX to cap on. For me, I think that'd still take an extra 10 DEX or so added to my TP gear, but that wouldn't be terribly hard to swap in.
I am only going to guess here so forgive me if this is wrong... but if i was to crit hit just once more in a fight would i do more damage on that one swing than i would do by having +5 STR for the duration of the rest of my swings in a single fight? what if i crit hit 2x more?? or even 3x more???... would that not be better than +5 STR?
You won't crit 2-3x more. Natural crit rate has a lower cap of 5%, and an upper cap of 20%, so dDex will affect 15% of that. Now, granted, the last 5 points of DEX are pretty clutch, since effectiveness per point seems to go way up as you come close to the cap...but that also assumes that you're right near cap already. At lower amounts, it's more likely to be a 0-2% increase, depending on dDex.
By contrast, pushing your fSTR up will effect every melee hit and every non-elemental WS. It effectively makes your weapon slightly higher damage, which will result in higher crit damage as well. Granted, that'll cap too - and more easily for 1h weapons to boot - but between the two stats, DNC (and Mithra in general) will behind on STR more than they will on DEX.
Basically, sans Atma at least, it comes down to that STR is nearly always helpful, whereas DEX is highly situational. Doubly so when you extend that to possibly having 2h melee jobs as well.