The main reason why the BLM levelling experience feels a bit more sensible is because it hasn't ever been reworked. So you can still see how each expansion built off of the previous iterations.

One downside to having different stat priorities on different jobs within the same subrole is that it means that you may have to remeld your gearsets differently to play optimally. Spell Speed on BLM is a bit of a niche case because BLM exclusively uses magic damage and has the lowest APM count in the game at baseline. Gish jobs in this game are always at a bit of a disadvantage because they still haven't bothered to merge Skill and Spell Speed into a single Haste stat, which really needs to happen. Now if we had a Haste stat that not only influenced your Weaponskill GCD/Cast time but also increased your movement speed, that could create some interesting choices.

I wouldn't use BLM as the standard for anything, though. It's the least played caster job despite having a significant dps advantage over jobs within the same subrole. It's also the preferred job of the director and producer of the game. I think if you had any other job with divergent design it would quickly become underpowered and ignored.