Speaking from personal experience... is literally to just improve wherever & whenever I could regardless of what job I play.
I remember when I was still very new and just realized how bad my performance was. I started small, from the easiest content and make my way higher as time passes by. Being able to see my own ### gave me the affirmations I need rather than resorting a feelycraft which are more likely to be biased than not. I want to know that I'm doing something -right-. It doesn't have to be perfect, but I also don't want a vague metric that is the enmity order list.
EDIT
There were only few times I remember seeing people giving away the fact that they're using one:
- That one NIN I was paired (as a RDM) in Heroes' Gauntlet back in 5.3. Homie complimented "Crazy dps RDM!" and literally wrote my number in party chat after the healer and tanks left. It wasn't a sour encounter but part of me surprised they just out'ed themselves like that.
- Late 5.5 when I was helping a friend into DRn grind. Somebody in the party asked "How's that WAR doing so much dps???", which prompts the rest of the members to ask how did he know and jokingly questions "What's my number? lol".
- Early in 6.05 when I was farming e8s unsync'ed as an AST. Several wipes happened because the PF couldn't meet the dps check. The party disbanded after the tank mentioned "We're not going to skip LR guys, the AST is doing more than two of the DPSes :/" and nothing else. Just gone like that.
- Same, in early 6.05 when panda normal first drops. The tank forces a group wipe and refuses any raises whenever somebody ignores pull timer. When somebody asks "is this barse?", they weren't shy to admit it was. This was the only sour encounter I ever had because I was tired that day and just want to get my weekly loot ASAP. One of the DPS purposely ignoring pull timer didn't help neither.