The lack of an effect from the pulse carbine's underslung photon grenade launcher is purely a mechanical fault of the 8th edition rules (and if, like me, you use older rulesets instead then there isn't a problem there).
In regards to Firewarrior armament, it's a matter of doctrine. Aside from the two exceptions of Pathfinders and Stealthsuits (and sometimes Gun Drones and Piranhas), Tau units are never supposed to be operating without support - if a Firewarrior team is operating on their own then something has gone horribly, horribly wrong. Contrary to recent popular army list ideas, Tau Fire Caste doctrine essentially places the Firewarriors as the core combat force, with all other elements acting to deal with anything their pulse guns can't handle.
Thus, Firewarriors already have all the flexible special weapons they need - they're called XV8s. The role of Crisis Teams is to act as troubleshooters, using their jetpack mobility to jump around from hot spot to hot spot providing special weapon support. That's where the 'Crisis' part of their name comes from. Other Tau units perform similar functions. So if a Firewarrior team ends up in a position where they need flamer support, then they'll just radio in a flamer-armed Crisis Team. If they come across an enemy tank, they'll markerlight it for a Hammerhead or seeker missile, and so on and so forth.
The key here is to remember that, despite what similarities they might have, the Tau are NOT a modern western style military. They are an alien force, with their own alien doctrine and their own distinct way of doing things.
It's also worth mentioning that the pulse weapons that Firewarriors are equipped with are already very versatile weapons in their own right. They provide enough firepower to effectively counter infantry, light vehicles and even tanks with careful targeting - in real-life you generally need a 20-30mm cannon for that kind of niche, but the pulse rifle and pulse carbine are both in the small arms size-range.
Finally, there's also other factors outside of combat to consider. Even if an ARC blaster can provide more armour-piercing capabilities at close-quarters, it's unreliable at any other range bracket and likely to be far more demanding in terms of maintenance. Remember, it uses a new kind of technology that's still yet to be fully proven, while the pulse rifle and pulse carbine are both tried and proven designs that have had every possible error and fault thoroughly logged and given an identified solution. Not only that, but every Firewarrior using an ARC blaster is one less Firewarrior that can engage enemies at long range, which is a crucial component of Firewarrior tactics.
Finally, mixing ARC blasters and pulse weapons adds unnecessary complications to logistics, and logistics are critical in warfare.