It's a cap on progression so the gap between people with long play times (e.g. 40 hrs a week) don't progress too far ahead of those with less time (e.g. 10 hrs a week).

You can bring a different job into Coil each week if you want. You are just not allowed to gear them up in parallel.

So the op is just complaining about a cap on progression, but want to make it sound dramatic, i.e. "Square Enix is going against their own design".

Remember, the armoury system allows one character to switch into different jobs, but it is still just one character. You can be a salesperson and a graphics designer at the same time, but you still can only buy one pack of beer when they limit it to "one pack per person". You won't go and say "hey I bought that pack as a salesperson, now I'm just buying a pack as a graphics designer"---but this is what you're doing right now.