But I already shown that they would take the same amount of time to apply? They are both 5.1 seconds.
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I think you may be missing my point in the reason.
They are not the same. Your DoTs do not all appear as soon as animation is finished.
In terms of when DoTs apply onto a mob, Miasma is the fastest cast/animation -> debuff appearance in your 3 basic DoTs. Next is Bio2 and finally Bio being the slowest.
When you have a vast mob of players either on an instance like ST/CT or in Hunts, where you WILL approach the DoT appearance cap or approach the actual 60 DoT cap (and you may not have all your DoTs up) switching up your beginning DoTs will guarentee you can see all your DoTs and they are all on the mob.
We have Cast time based DoTs, like you said with one Instant DoT (Bio), we also have a delay in system on our Bio making it one of the slowest appearing DoTs in game. Compared to other classes which for the most part do not suffer this, and most other classes have Instant Cast DoTs.
There is a difference between:
casting 2 skills and 1 Instant,
and
casting 1 skill and 2 Instants,
and the fundamental difference is in this particular circumstance when both you can become visually challenged on seeing the duration of your DoTs, or miss out on a DoT entirely, if enough people are putting up DoTs immediately, in those seconds your casting those two skills.
In going through 2 casts +1 instant, you have a high chance of not being able to see your Bio on the mob, thus losing track of when it needs to be refreshed or possibly not having it at all because you hit the DoT cap, if enough people are putting up their DoTs at once.