So, when I got Mysteries for 5E, I could not for the life of me figure out the point of Rotes. They aren't that much easier to learn than formulaics, and if you know the formulaic you can just take -15 on your casting to get Still and Silent, instead of having to divide the whole thing by 2.
I think I finally figured it out. Jerbiton.
They already value high Artes Liberales scores, so there's that much advantage going for them. Most importantly? Rotes don't botch. Jerbiton like to hang out in cities, where you've got a Dominion 3 most places. Those extra botch dice are very dangerous over time.
It gets better. Assuming an Artes Liberales (arithmetic) of at least 6 and the lesser book bonus of +4, you only need a base lab total of 15 + those two to learn level 5 Rotes. If you can manage a Rote Casting Score of 20 (regular Casting Score + Arithmetic), you can cast the applicable level 5 Rote - which already gives you Silent and Still - despite a level 3 Dominion.
All in all, very nice for Jerbiton trying to keep a low profile.
And now I have an idea for a Jerbiton Mentem specialist who masters Rotes to get Fast-casting. All he needs to do is meet your eyes for an instant and you're hit with something or other.