But it's NOT constant. In my example, the two rogues- Rogue-A and Rogue-B- were both balanced to average 1000 hit per attack. Rogue-A did it with 0 base crit, Rogue-B did it with some base crit.
The base crit on those moves was never free, and it raised the mulitplier coming from crit from, say, 1.0 to 1.15. This means that while you can still add 10 to the multipliers, it should be obvious that going from 1.0 to 1.10 is going to give you more bang for your buck than going from 1.15 to 1.25. Hell, going from 1.0 to 1.10 is 10% more damage- going from 1.9 to 2.0 (going from 90% crit to 100% crit) is only a bit more than a 5% boost. The damage increase is smaller.
But instead of being DESIGNED that way, you could be that way from stats. If you have a massive stack of haste rating, your crit rating will be better than haste, etc. The point is that having stuff baked in reduces the value of that, if you have a rating that just adds to it additively.
Can you tell me what is confusing about this? I'm clearly failing at this explanation