In addition DOTs prevent additional repeat damage, like overlapping Brand passive of Malz E, which prevents maximum damage potential without further spacing spells.
And while true, because of this added counterplay, DOTs typically get higher overall damage than up front damage spells to compensate (or rather, they're high and made into DOTs to compensate).
Like the aforementioned Brand's 8% max health damage or Malz's 320+.8 AP E or 55+1.3 AP R.
If those damages were up front they would be absolutely absurd. The reason they aren't is because they're DOTs, but that doesn't mean they're the same as other spells, except just DOTs. Nah, they're still absurdly high damage.
I'm sure Riot gets complaints about anything people deem too high damage. Remember when Teemo shrooms were 600+.8 AP plus Liandry's burn right at the start of season 3? Yeah, despite them being technically dodgable (their location discoverable) and also the DOT being over a huge amount of time, no one liked that.
Also, speaking of Liandry's, while in a vacuum being a DOT is a strict drawback, with items, having a DOT is a strict bonus, since it increases Liandry's burn duration and Rylai's duration.