Doton on Bosses, am i missing something?

[mosaicex]

It does 8 ticks of damage (yes, 24s dot has 8 ticks (24/3 = 8, I do not know why you got 6 instead), if you don't believe just test it on training dummy), so 360 potency in total.
Anyway, most NIN who use Doton on single target use it as finisher for their Ten Chi Jin. Fuma (280) + Katon (250) + Doton (360) = 890 Potency in total
If you do Fuma (280) + Raiton (400) + Suiton (130) = 810 Potency in total.
That being said, I do not religiously play NIN so I can't really say how much Meisui Suiton worth in grand scheme of things, but that's the gist of why would people use Doton on single target.