The reason you want Bio last in your DoT application sequence is that it is instant-cast but initiates the GCD, meaning you can use an off-GCD ability (Bane or Fester) while your other skills are "recharging". It also lets you start moving to dodge something just as Miasma finishes casting and still get all the DoTs up. If you were to put Bio elsewhere in the sequence, you would have to wait for its GCD, then cast the remaining DoT(s), then use Bane or Fester while you could have been using an on-GCD skill and the enemies have a longer window to interrupt your DoT applications.