I've been playing this game now for about 7 months, in that time I have never bothered using anything other than Mender Merchants to repair my gear, although the option to use Dark Matter's have been available to all who join FFXIV at Level 1, it's something I always thought there would be no need of.
Earlier I noticed how cheap
Grade 6 Dark Matters (for Equipment levels 1-60)
were, at 120gil a piece they work out cheaper to repair...