I speak more in cases where the DPS is going slower than expected, which is when I will then speak up. An example would be in T10 of 2.X; we were often not meeting the DPS check for the second set of adds at first. When it continued, I noticed our SMN was not Bane-ing DoTs onto the other adds; I asked him if he was Bane-ing, and hey presto, next round when he used Bane, we cleared the DPS check. Another example would be a Brayflox I ran; the BLM was a fresh BLM, so they weren't very experienced. When it came to AoE pulls, he wasn't using Fire II or Flare effectively (I believe I saw his rotation as something similar to Fire > Fire II > Fire III > Fire > Fire II > Blizzard III > Freeze spam). I pointed out the better rotation (Fire III > Fire II until low MP > Flare > Convert > Fire III) and they instantly improved.
When it comes to multiple DPS (typical of DF trials), it comes down to a comparison level as well; for example, Bismarck NM with two DRGs; one was applying DoTs and using BotD, one was not. It's not 100% efficient (I don't think I ever said it was? -goes back to check previous answers-) but after a few runs and gauging the DPS, I can typically tell who is causing the wipes/not pulling their weight sufficiently. The bolded part is very important; if the encounter is cleared in an adequate time, I won't be as active in identifying flaws.
When it comes to most DF dungeons where minimal healing is required once I know the pace and environment, yes, it has minimal repercussion to my healing style, and I can gauge how others are performing; I generally get the overall vibe from the first couple pulls of how the DPS are, and then I start looking closer. If we're talking endgame raids in savage, no; I am currently learning A3S, and right now I'm trying to get myself comfortable before I start going beyond. I am doing that already in AS1 and AS2 however; there are a few downtime periods and spike moments where I can see who's not picking up their slack.And all this mind you with no negative repercussions to your own play, that is assuming that you are playing at the highest level capable for your class given that you are situationally aware enough to accurately and consistently gauge the performance of the 7 other players in party with you with zero margin for error.
You have to keep in mind that the most typical scenario of where I can take the backseat driver is in DF, namely dungeons and trials. I would assume that the topic branches not just within progression and relevant content, but across all contents to even guildhests (maybe not that low of a spectrum, but most dungeons). In your last example, I'll instead point out using the Mudras for additional DPS rather than for Huton. Also, it has to be below adequate performance for when I speak up; I have had runs where the feathers weren't being killed fast enough in Alwaysreap (Yes, the DPS were targetting them, though the BRD was not in WM and only using Heavy Shot).Saying that sometimes you're able to spot which DPS may or may not be under-performing (with no accurate way to verify mind you) is all well and good until it matters, at which point you have zero way of providing any meaningful feedback or assessment. saying to the random DPS in ex roulette "are you sure you're doing your rotation? I didnt hear you pop armor crush" isn't really going to hold any water in any realistic scenario.
It wasn't meant to be said to counter the subject at hand; I am neither for or against parsers. I have my own judgements on them, but my typical view is: make mine not your enemy, and I shall not make you mine enemy. Guesstimating will have mistakes; I will only speak up once I'm sure that the DPS is under-performing, usually with what I have observed. As long as I'm polite and offer tips, the DPS usually responds favourably and improved the next pull/round.Let me make it clear that I'm not trying to jump on you here, while the ability to see and guesstimate these things is great, your comments don't lend much in the way of credibility to the subject at hand. due to the not so practical nature of a DPS basically needing someone else to keep an eye on them, and the lack of any meaningful consistency.
Actually, I think I pointed this before; I also look at the status bars of enemies and players themselves. On the topic of melee; I will notice if a DRG isn't applying Heavy Thrust or CT/PB. Especially if I'm DPSing as well, and have the enemy targetted; movements of the DPS also play a part in confirming whether or not they're playing properly, as all melee have positionals. As you said, it's guesstimating, and I only speak once I know they aren't pulling enough weight (I can understand missing a PB every now and then due to DRG's timing with BotD, but there's a point I start questioning 'Why?')
As I've said, it's not for everyone. I've played most jobs/played with enough people with other jobs that I can pick up on these things. In high-tuned content such as Savage, I can only do so during a few downtimes and phases in A1S and A2S; currently, I am incapable of doing so in A3S because I'm still learning how to save my own ass while doing my own sufficient HPS and DPS.