I think of a few cases where prismatic is preferred over just any elemental.
Mother Slime's Tooth trophy needs careful adjustments to elemental damage. You want to make sure all of them will KO her at the same time. The general rule of thumb I use is 99 prismatic or 149 prismatic, depending on whether I have too much Muscle or Moxie.
The other is one of the flickering pixels in the Crackpot's Jar. Of course, that's a one shot in your entire life, but it's handy for that one time only!
Bugbears, once one of them has been banished, are easier to take out with prismatic damage. Especially 6-kiss bugbears with their hard cap of 75.