The opportunity cost isn't dictated by whether an action is AoE. It's dictated by whether it's dps efficient. Circle of Scorn and Salted Earth are dps efficient in both single target and AoE. Dark Passenger is dps efficient in AoE, and under specific conditions, can be dps efficient in single target. Decimate is dps efficient if you have two targets, and Quietus is dps efficient if you have three. Flash and Unleash are never dps efficient.

If you're talking about hotbar space, it just comes down to how many different situations an action can be useful in. That's generally why certain jobs tend to come out ahead, by virtue of having less "niche" actions and more multi-function ones.