Hi there, I see your concern but let me tell you something from experience, A person that leveled any class through leveling all the way to 50, STILL might not know how to play their class. I have darn well experienced this many times, but leveling something 1-50 does not equal knowing how to play your class.
As for introducing a new job to a class that you already have at 50? Thats fine too, you still have to put time into it to open all the job abilities by spending time grinding its story quest. So you still have to put time into the new job, just like you would leveling it. Its just through story fights, rather than grinding/dungeon/fate parties.
You may do the first quest and open a new job and have it at 50, but unless you put the time in on the quests, you wont have the abilities open, whereas some one else might have.
Another reason why this is an ok system. Say you want a pet class but want to play a healer role. Well, take up arcanist, then open scholar. Dont even worry about smn. It lets you specialise without having to spend time on a job you dont want to play. I'll level cnj for cure for when I solo, but i'll never play whm lol
lets take your samurai off of lancer idea. if they do introduce samurai, some people may love it and switch over, they still have dragoon but dont have to play it. It just gives people more options, which is always great.