User blog comment:DeMi189/Katarina guide - Why? Because I can!/@comment-4945936-20120911025908/@comment-4717259-20120911160847

The reason why I take AP/lvl runes is because they have better quality than the MPen ones, only the marks are better as MPen. There are runes which are better than the others, in case you don't know, I'll write it: Marks are best as APen and MPen, Seals are best as Armor, Glyphs are best as MR, MR/lvl and AP/lvl, and Quints as AP and MS, you'll find the rune ranks here: http://www.mobafire.com/league-of-legends/build/rune-tips-what-secondary-and-primary-means-and-more-24255#chapter5

I don't think Armor Seals are worth while for a mid champion, as 90% of mid champions I encounter are AP, the Armor doesn't scale per lvl, and Armor/lvl becomes better after level 6. I should need Armor at about level 9-10, when the teamfights begin, at that point the flat Armor seals are already fallen behind. Unfortunately, Armor/lvl is not very good to take. I'd take HP/lvl or Mana regen/lvl Seals, but I play lots of manaless AP champions, and as I said, I'm really aggressive, so AP/lvl Seals turn out to be better for me. And I think damage is better than surviving minions :)