Well as kalbear pointed out you can't talk about a stats without taking into account other stats. Basically arpen stack with itself so untill you reach the cap it's better to stack it. After that you want to stack agility simply because it's our best stat also if it give us crit and ap.
As for crit "soft cap". Actually:
you can reach 100% crit with yellow attacks (but it doesn't mean that every yellow attack will crit).
you can reach: 100% - glancing (24%) - miss (0-8%) - dodge (0 - 6.5%) with white attacks. So the white crit cap is from 76% to 61.5% depending on your hit/expertise.
What you can easly see (look at
FeralbyNight (Cat & Bear simulation tool) )
is that the increase in cp/waste is about linear from 30% to 60% crit while after 60% crit there is a power increase of wasted cps. So yes, crit/agility value is nearly stable untill you reach 60% crit and after that it goes down, but as already told kalbear cps generation is only a small part of agility value. For istance if at lower tiers 1 agi is around 2.6 AP, in very high gear and with for istance 70% crit (not possible atm) 1 agi can be around 2.3-2.4 AP (still pretty high).
The only thing we should take care (for future tier levels) will be the value of hit/expertise. The value of hit/exp increase pretty quikly (from 1 hit = 1.8 AP in blues to 1 hit = 2.4 AP in full t8.25) because it's direclty proportional to damage done. So we can easly assume that in t9 or t10 we can have a situation where after reaching arpen cap we will reach hit/exp cap and THEN we will go for agi, but at the moment that's not the situation.