Making jobs unique has nothing to do with adding more jobs or not; it has everything to do with this
1. Strict dps requirements
2. Strict healing requirements
3. Strict time requirements (every second matters in savage/ultimate)
4. Doesn't allow for experimental gameplay
Anyone that's played mmos long enough knows how classes are balanced around the hardest content. Thus the games are usually seen as unbalanced like FFXI because high end content consists of the same classes. Or they're seen as boring and 1 big blob of all tanks having 123 + self heal and 1 burst phase like War/Drk because they don't want to keep any jobs out of high end content. Stormblood was the last unique expansion, Heavensward was better at this, and A Realm Reborn was perfect in terms of job identity and creativity.