If you want variety, why would you ever want branching jobs? If you're really after variety, you'd want a completely new one because that would be... y'know, more varied, rather than being the same thing with a handful of different skills.
Branching jobs stifles variety.
If I put ketchup on one burger and mustard on the other, it doesn't change the fact that I'm still eating a burger. Some of us would rather they give us a rack of ribs and a burger than two burgers with different sauces. Because some of us prefer actual variety.
Red mage DPS branching from Gladiator, for example, is a bad idea in itself. Rampart is a tank skill. Flash would be useless. Convalescence is a tank skill. Provoke would be useless. Shield Swipe would be useless. Awareness is a tank skill. Bulwark is a tank skill. Sentinel is a tank skill. They'd be missing out on a lot of skills that are defensive skills that are useless to them because they're DPS and need offensive skills.
It's faster to list the things that ARE useful from GLD for a DPS. Fast Blade, Fight or Flight, Riot Blade, Shield Bash (assuming they have shields). Everything else is either defensive or has increased enmity.
Not to mention that the extra threat of Savage/Halone would work against them. Because remember, scaling is a thing. A RDM scaled below 30 in a low level roulette would be no different from a PLD. That means they'd need to completely redo how threat works in low level dungeons just to make it work somehow that the RDM wouldn't just be ripping everything off the tank. The only other way would be to literally have your RDM play as "Fast Blade, Riot Blade, Fast Blade, Riot Blade, Fast Blade, Riot Blade" for anything below 30. Which isn't much of a mage, now is it?
Or they could just create new jobs that don't branch, giving more variety and not having to rework the entire game.