I always say 'Heyos'/'Hellos'/'Hello, hello', etcetcetc and 'Thanks for the party!' (......unless I've been put into a rather foul mood during. Then I guess I'm not thankful for the party >_>). I will pretty much mirror the mood of the party - if it feels like it's a light group, I'll chat/joke. If it just feels.... busy or heavy (I don't know how to describe that feel) and silent, I'll usually stay silent also. If there's a wipe, I might try to lighten it with a "So close T^T" comment or a "Drat >.<" or something....a "@____@;;" if there's a close call or the fight was particularly hectic.
If people are chatty, I'll be chatty - in moderation, considering there's a 95% chance I'm playing healer.
If people seem like they don't give a shit that you exist, I won't say anything.

I don't feel that "you'll never see them again = there's no point" is the best way to look at it, but to each their own. It does add amusement and liveliness to a dungeon - no matter how many times run - when the people around you actually have a personality.... or at least polite enough to say hi. Makes a grinded dungeon more interesting. The best DFs are the ones that are good at both - chatty without getting in the way of the doing. In the 8 mans, the chatty ones stand out, to me and it's usually welcomed. It changes things up. You may not see them again, but at least the experience was a good one. Or, at least, different/unique.

In all, I'd rather have a friendly, comfortable group over a "godly" robotically silent group.

I've also noticed the sudden lack of commendation giving. I'm not really the type that thinks they need to be commended just for healing, but after pulling more than my weight in something and keeping the party from what would have been a wipe, somehow pulling off a miracle and what ever else.... nothing. Awesome. I guess I shouldn't try so hard if no one cares what you're doing for them. I always commendate the person who was either most friendly, did their job best, or seemed to go above and beyond. The people that shine. That is, when there's people left to commendate. Everything has become about getting out of the dungeon/trial like your ass is on fire. And I blame the weapon grind for it.

I'm sincerely hoping SE doesn't add in any more dungeon grinds - it's turning the playerbase impersonal and in a hurry. I doubt thats what they had in mind for the playerbase, but that's what it's become.

Feels like NYC (or worse, Penn Station) at 5pm rush hour - 'don't care who you are, gtfo of my way.'