If you think about it, several characters can meet (or possibly exceed) Master Spark's damage if you actually get them to 100% ATK/MAG buffs first. It's just that Marisa's Concentrate makes it the most easily viable for her to do it. Especially considering when the boss is weak to an element that is not MYS, or resists MYS.
Well, and Master Spark is still probably more powerful then most things in that case, although the fact that after 1 shot... that's all, folks! Which means only superior because of Concentrate and for FINAL PHASE MUST NUKE INSTANTLY.
You don't understand, Master spark gets more and more useful later on. Because instead of being that one nuke you use at the end of the game, pretty much immediately after Yukari, you can use it whenever you please, and Marisa will regen her sp back to 1600+ before the battle is over, to be used again. and the 2nd time, you wont have to rebuff her mag from 0 to full again, it'll already be at 60-something. Marisa is simply underwhelming for Yukari in general, because yukari is a relatively squishy boss for how hard she is (and how late game she is), combined with the fact that she's almost sure to djinn storm RIGHT before phase 3 which is when you no doubt wanted to horde all that sp.
As for patchy-level mnd, that's just assuming you don't make patchy's mnd grow as well. spend some skillpoints for patchy too and it'll grow beyond anything anybody else can hope to reach with equal commitment. Patchy pretty much solo'd the floor 27 boss for me.
Yukari herself is not a character I used on my first playthru because my party was already more or less established. Her stats are nice, and her 2 support spells are indeed good. However it's worth mentioning that you should never really count on her other spells. I had like a million or 2 skillpoints saved up for Yukari when i got her, and I poured them all into just her as soon as I got her (I made a backupsave in the event she dissapointed me), and gave her good equipment. But the damage I got from her spells (and the rate that speed debuffing one actually debuffed) was extremely dissapointing...Though I guess her chen+ran one is probably great if you have those characters in your group.
Having a 2nd character to hakkurei barrier when Reimu gets destroy magic'd or something would be very nice.
Still though, I honestly would not dump Marisa for anybody, or Reimu, master spark and that aoe heal are one of a kind abilities that nobody else has something remotely similar to. Master spark really does seem kinda underwhelming at first but it gets better more and more faster than everything else gets better due to its nature of being more frequently used in tough battles later on. And as I mentioned every master spark after the first doesn't require nearly as much concentration time.