Okay, so I'm going to focus on a particular phrase here that gets thrown out a lot in these kinds of conversations: "playing optimally"
What does this particular phrase often boil down to? Typically it's, "How high is your DPS number?" This means pretty much everything you do in the game boils down to a single measurable value that is then used to determine your competency at a particular role or job. The problem with this is that there are a myriad of other factors that come into play when doing content that should be considered and many times that comes down not just to your own skill level but the skill level of the people you are playing with.
There are 2 general scenarios you can consider here, the first of which is what many people tend to default to: playing in a group with the same people as a static where everyone knows what each other is capable of and what they can do in terms of their role. When you are playing under this scenario you can justify expecting the optimal scenarios that the optimal setups and strategies require. Now, the fact of the matter is that for the vast majority of content, in order to complete it successfully, you don't need the absolute optimal performance. The trouble is that so many people view even a slight variance from this ideal as directly negatively impacting them. Are there instances where someone is doing an absolutely terrible job at their role? Yes, but that doesn't meant not being 100% optimal all the time is bad either.
The second scenario, and the much more common one for people to deal with, is running content with PUGs. In a PUG it is generally the case that you have no idea how good or bad the other players will be. Likely you've never played with them before, likely you will never play with them again. You don't know as the MT if the healer is going to preemptively shield or regen you before the tank buster. You don't know that the OT is going to shirk onto you so you can tank in DPS stance. You don't know if a DPS is going to mind their aggro meter to help assist a tank in DPS stance not need to use Halone or other things to maintain them enmity.
Once again, not all content is high-end, Savage raiding. The vast majority of content isn't going to be affected by a 100-200 DPS decrease from the tank. It may take a few minutes longer, but generally you won't wipe unless the tank is just demonstrably bad and can't maintain aggro or doesn't even try holding more than 1 mob. Optimal builds, optimal strategies are effective and efficient only in optimal scenarios. The chances of finding yourself in an optimal scenario in a PUG is abysmally low.
TL;DR: Optimal circumstances generally don't happen in PUGs, so you can't count on them happening. Doing what works to get you through content despite being not entirely optimal is better than your being optimal and your group failing because they can't keep up or adapt to the situation.