There is no need what so ever that I can think of to add new classes but
1-Access to new abilities (3-5 at best per class, with the majority just being 3 not counting WSs but who equips those anyway?)
2-Access to new jobs which is what people want much more than the class.
3-Access to new weapon types. (Purely aesthetics)
However Jobs can have abilities that can be crossed, jobs themselves are the jobs people make up classes to unlock and without being locked to one weapon as enforced by the armory system you can have many more weapons with their own weapon skills.
Mace/Morning-star Paladin
Hammer/Club Warrior
Tun-fa/Staff Monk
Some content could have mobs weak to blunt damage, or stabbing etc etc.
Also adding just a job that you can unlock and play @lv1 is more satisfying that adding a Class then later adding a job (I highly doubt we'll get the job WITH the class at the same time) and just being a lv50 "that job".
The original armory system was based around fatigue, had total freedom with no limits on abilities, no jobs and many other factors and aspects that were either scraped/changed/removed/added into the current one.
Switching on the fly is fine and dandy but it requires more limitations just like classes and jobs got limits added to them so should this, since the ability to be anything at any time created situations like job stacking, mid-dungeon party changing. (Easy mode)
As for removing classes this is nothing but positive and I dare say and improvement over the current gimmicky system that the new team got stuck with from the old. adding limitations and jobs is their attempts to get rid of the plague of all classes using the exact same abilities and being basically the same class with a different weapon.
Remember needing THM50 for emulate just to be able to tank on GLA? yeah they adjusted that and now Jobs are limited to just 2 sub classes and the highest you need a secondary class is 42(ARC) for chameleon and with the job system and not all of them need to, which is an improvement. those that just want WAR will just need GLA22 and PUG14 for the abilities (but MRD30 GLA15 to unlock) which is a lot feasible.
Why not take it a step further and just remove classes?