[Baalz]

Well I can see your point, but the way I see it is like this.

Astral Corruption/Utterdark/Burden of Time/etc. - (for the sake of argument) screws everybody but the caster roughly equally.

Arcane Nexus/Forge of the Ancients/etc. - Benefits the caster and nobody else

In either case the only change in advantage is between you and the caster. It doesn't matter if the caster is directly benefiting or screwing everybody else up, the effect is the same. Casting something like Purgatory though, if you're Ermor would be a completely different thing.