I think that is one of the issues, it's consistent. This means the player can without a doubt, literally control the tide of the fight just using common sense for the most part since the turns will be pre determined, there is no surprises generally speaking. No real reason for the player to think, since you can play auto pilot due to going 1st being usually more influential than not.
Whereas if fastest "X" character isn't always 1st, well now the player has the decide or plan differently. Of course in XS this can be changed purely with the "Boost" mechanic, allowing for turn skips with a cost. But since that also applies to the enemy, that's fine.