As a veteran of the series as a whole, I noticed immediately that DRK took a lot of elements from the Mystic Knight/Rune Fencer Job. It's an older Job dating back to FFV, long before DRK became truly defined, a Job which fought by enchanting their weapons with various spells, usually from the black magic spell pool. Dark Knight in FFXIV isn't a far cry from that, and that may have been what they were trying to evoke with the change. I still dislike it though!
Couple of points: First off, just... turn off Darkside? Maybe ask your BRD or MCH for Refresh? It's not that hard to do. Secondly, the devs weren't limited, if they were, we wouldn't have had a close to 40s break in Alexander Prime's encounter when he decides to use Divine Judgement. Finally, DRK is stronger in PvP now than they were in 3.56, and DRK now has a specific niche, they're anti-healers and pull squishies into the pack to slaughter them.