I have not had an issue with either queues or players leaving once the instance has begun. After 3.5 hit, the queues were understandably a bit longer, but they are back to normal-ish now, which for me as healer averages 5-10 minutes. If one of the FC tanks comes along, they are nearly instant.

Now of course, I am not going to object to re-purpose these dungeons to make them more relevant, but there is a lot of reasons to come to these dungeons if you run content outside of ex/raid. They provide really good exp if you are leveling from 50-60; they are needed to unlock 50 roulette, which will provide a daily boost; rare mats can be obtained here, including desynth items and torn copies for orchestrion scrolls; some are just seriously fun to run like Brayflox HM, Sastasha HM, Halatali HM (feels like an advanced HoN). I also use them to warm up for more difficult content, and there is almost always a new player which provides a bonus.

When Stormblood hits, they could make these dungeons more relevant by making at least some of them a requirement before progressing into SB. Won't be an issue for anyone who has already completed them.