I have two main grievances with Ghost:

1. It is a real pain in the ass to have it inflicted on you by an enemy; at least with Dark Ghost you can remove it if you have a cleanse silver effects skill or effect, but the only way to be 'immune' to having Ghost cast on you by an enemy is to somehow cast perfect corruption/block beneficial effects on yourself. If Ghosted by an enemy (e.g. if you kill a character that has the Soul talismans) your only hope is to extend beneficial effects on your team until the end of the battle and hope the enemy don't have a permadeath skill. Also, on the topic of Soul talismans I often see Ghost cast on 2 or even 3 of my team if I kill someone wearing it in Gauntlet; not the single enemy as described in the compendium...

2. Soul Protect takes precedent over other revive skills; if a character dies with both a revive status and a soul protect status, it will Ghost rather than reviving. I get that one has to trigger first but I would much rather have the revive trigger first. This is one reason I avoid using characters that cast soul protect on all allies as part of their passive as it basically makes any other revive useless.

In terms of dark ghost extending enemy life it can be useful is utilised alongside permadeath or perfect death, but its rarely useful as a skill on its own unless you have no way of damaging said enemy and casting a guaranteed slow death is the only way to kill it...