It's still optimal to use the "correct" level of Dark Matter for your repairs if you don't have money coming out of your ears and you want to save some gil. Grade 1 matter is MUCH cheaper than Grade 5 matter. I don't have grade 5 Dark Matter in abundance because I don't farm Ifrit, so this OPTION to be able to use Grade 5 matter in place of the lower levels ( and grade 4 for 1, 2, and 3 and so on) is great in case you're short on the lower level matters. You sacrifice money for convenience.