The same I do right now: parse myself and have fun.
I'd only call out low DPS in random groups if it causes us to miss enrage windows etc.
A.k.a.: preventing the clearing of content more than once.
Which, in my decade+ of experience is rarely necessary in random content (savage/extreme level excluded) since after the first wipe most folks wake up and take it seriously and stuff dies.
- - - Updated - - -
In expert: yes. I've never seen anyone do that though.
In MSQ? Not really. 90% of the runs duration are unskippable cutscenes, the rest dies so quickly that it can hardly be called a fight.
- - - Updated - - -
I've yet to see that in real expert dungeons. You get the odd player that has low output but I've never encountered actual AFKing.
Would be way too visible in a 4man group anyway.
MSQ is pretty much the only place where I encountered semi-AFK people because 90%+ of the run is just cutscenes.
Heck, I myself have been afking the war machina up to Nero part where everything dies instantly if 2 players shoot it.