It's a time-saving feature when people are killed as a boss is being defeated but shortcuts solve this too. There isn't any other notable benefit to having a release option when downed in an instance: releasing to be ready to retry is pointless as you'd still have to wait for the last players standing to respawn and catch up.

Trials don't allow players to release, probably because there's no safe starting area, and they work. I understand the frustration of being raised and dying nearly immediately, that poor or unsupported healers might forget/be unable to heal you, but releasing just to avoid it doesn't help anyone. Releasing out of spite might be justified but I'd rather a vote to kick a problematic player or end the instance than passive-aggressive acts. If you're going to stand outside the fight then please leave the instance.

I'm in favour of removing the option to release because it forces us to deal with problems and keep fighting or leave.