You're neglecting to mention that a DoT is also supplemental damage. Someone could plausibly ask for a DoT just because they want it to contribute to RDM DPS output and there would be nothing wrong with such a request, despite it being just periodic damage and offering no additional functionality. Of course, there is the matter of how the DoT is designed (because we don't want a repeat of Fracture), but the act of making such a request is not inherently wrong.
Personally, I wouldn't implement a DoT that gives high mana ticks, as not only would it be awkward to use, but would end up being a slower Manafication. I'm more a fan of a "reapply every 45-60s to get 2/2 mana per server tick", as it would be a benefit over time between the supplemental damage and the additional mana gained. Depending on circumstances, this would make the difference between going straight into the melee combo or having to cast one or two more spells (imagine being at something like 76/69 with no Verfire/Verstone Ready; Jolt + 11 mana verspell won't let you do the melee combo, but if you factor in 2/2 mana generated by a DoT you applied earlier in the fight you'd generate the mana to take you to 80/80 by the time you finish casting the 11 mana verspell).Meanwhile, in the case of Mana generation per tick, one can readily imagine that you might put out a situational (at least, if rapidly-ticking) DoT before you'd otherwise overcap in order to allow for, say, 5 back-to-back Enchanted Moulinet casts.