You are taking things out of context.
Again, I reiterate
"No one but the most hardcore elite of elite expect people to play like Gods. They are expecting them to know their role + class/job well enough to help defeat mobs in a timely manner based on their gear score."
I personally play with a sub-optimal setup because it suits my play style. For example
-I main WHM and I use a Crit Build, regarded as the worst build possible for a WHM to play. I am planning to build a Thyrus Novous that is ACC > Crit. Will this affect my performance? YES. Can I compensate for this with my skill sets? Yes! Will I ever achieve 100% super God-like play? No, because I am intentionally hampering myself to suit myself. However, I am NOT hampering the people I play with because of this decision on my part because I can compensate for the sub-optimal gear selection.
-When I play BLM, I start my single target rotation with Thunder (Not Thunder II or Thunder III). Optimal rotation for BLM begins with Thunder II followed by standard fire rotation. I am intentionally going a sub-optimal route because I enjoy going into Astral Fire III as soon as possible. Using Thunder will allow me that 0.5s faster. I also go from Flare into Tranpose into Thunder into Fire III (usually not capping MP) to go back into my fire rotation. Again, sub optimal, but it suits my style and hardly hampers my play for the content I want to do.
However, what is being discussed in this topic is not SUB-OPTIMAL but SUB-AVERAGE. You need no parser to know when someone is playing sub-average. You may need some knowledge of other classes to help you understand what they are doing wrong. However, in some respects all you need to do is look at the enmity accumulated on a boss to know how well or poorly a player is doing in respect to the other DPS in a light party.
Now, this isn't to say there are other factors involved. One can basically glance at the HP levels of the two DPS to determine their gear score. If one BLM is at 4.2K HP and the other is at 3.2K HP, then you can tell that one BLM's left side piece far outweigh the others. But if two classes of similar ilvl are paired side by side and one has a very distinct enmity lead, then one can infer that the DPS in question is providing a heavier contribution to overall damage between the two.
And then, if both DPS are playing poorly, this can lead to death in certain instances (Demon Wall being one of the prime examples back at the onset of the game). No one is asking for maximum DPS, maximum epeen factor, "God-like playing". But people are expecting the DPS to know enough about their rotation to over come these hurdles. If you cannot even perform this, and these individuals are not willing to listen to advice (when presented nicely, poorly presented advice is an entirely different can of worms), then this particular individual is now hampering the time of at least three other individuals.
People are not asking for you to do MAXIMUM damage. People are stating that they wish to see you do damage that is considered normal and respectable for your role.