They will not get the full effect of an enemy debuff, nor do their skills fit in the tokens (like mnk wars, blm all have their own set of charge counters), nor do they apply with any debuff length.
the GCD heavily favors direct attacks, for pretty much every class except ACN because that's all ACN has and even then not really, when the mnd/int down debuff is 10 seconds.
The point of mnk is to grease up, the point of blm is to astral up, the point of a war is to wrath up, etc.
Very few jobs will pick a dot in their rotation unless it's all they have.
You use Dot because they charge the Limit break, otherwise, they're crap most of the time.
Dot can crit on the initial but is applied separately on the dot potency.