It’s all down to math. Over the course of a long fight, it is reasonable to expect 3% crit to happen during 3% of your attacks. With crits being 200% damage dealt, it’s reasonable to expect that to come down to ~1.5% damage increase. The longer the fight, the more likely you are to reach those results.
However, there is such a thing as diminishing returns. You talked about having a low crit chance… but did you know that’s when crit’s return is the highest? Jumping from 0% crit to 10% crit should be around ~5% increase to your damge. Jumping from 10% to 20% will not yield the same increase. Yes, it’ll be ~10% increase to base damage, but now you only got ~4.7% from those 10% crit, so even though you doubled your crit, you didn’t get double the benefit. The higher you go, the less it’s worth, which is why stat weights are constantly evolving and you should keep simming to be accurate. The reason is because each stat is additive, not multiplicative, but each stat multiply the others. If you have low versatility, you get close to 100% value from versatility stat, which might not be enough if Versatility is terrible for your class, but if the stats are close enough, it might just tip the scales.
The only time you want to hit certain breakpoints is when you have class mechanics around it. For instance, Holy Paladins used to double Holy Shock’s crit chance, which meant that at 50% crit you had a 100% crit chance, which is reliable, something healers like. At the same time, Holy Paladins had cooldowns that increased crit, which meant that being too close to that permanent 100% holy shock crit meant you’d be “wasting” crit during Avenging Wrath. But for DPS it’s really just a numbers game, and the longer the fight the more RNG tends to just boil down to its own percentage.
In general you just go with the highest iLvl. According to bloodmallet, the difference between ret with perfect itemization and ret with worst itemization is ~4% DPS. That’s about 8 iLvls worth of stats, meaning that any time an item has 10 stats more than your previous item, it’s an upgrade. Also according to bloodmallet, the perfect itemization would come from 30% crit, 50% haste, 10% mastery and 10% versatility. If your question really is about PvP crit would be worth half as much, I’d imagine it wouldn’t be worth it for long fights. But PvP is also about short skirmishes, and having crit ups your burst potential, even if unreliably. PvP is less of a numbers game.