Miasma's damage contribution on initial tick simply isn't enough to overcome its tick damage loss vs Bio and Bio II, after just one tick it does less damage.

Again, fester is worth it only if the target needs to die asap, you lose overall dps using fester on a target that would die in less than a couple GCDs however.

Its pretty easy to calculate when bane is superior to fester, and that is since your dots do 95 potency a tick, and fester is (max) 300 potency, it requires ~9 target*seconds before bane is superior.

So bane > fester at

1 additional target - 4 GCDs
2 additional targets - 2 GCDs
3 additional targets - 1-2 GCDs
4 additional targets - 1 GCD