Like the HQ and Elites sections, the Tau Empire provides just two choices when it comes to Troops. Unlike the HQ and Elites sections, however, where there is a single, obviously superior choice, the Troops options aren't nearly so sharply divided between 'essential' and 'wasteful'. Unfortunately, that's not because the two Troops options Codex: Tau Empire provides are so good, but because they're both fairly mediocre.
I'm going to focus on the Firewarrior squad, because it's a 1+ unit and because, well, it is the Tau Empire and not the Kroot Empire. But don't think that means the Kroot carnivore squad is all that impressive, or isn't in need of no small amount of fixing come the next edition. It's just that the Firewarriors need the working-over more, and ultimately they're the more important unit.
So, what is this unit for? The obvious answer is 'holding objectives'. Two out of the three basic missions in 40K involve holding objectives, something only troops can do. And the Kroot, with their T3 and no armour save, simply cannot hold an objective for anything decent length of time; they'll just get shot off it, or worse, their large unit size will make them easier to catch in an assault. Firewarriors may have the same toughness, but a 4+ save gives them a great deal more staying power. But as it stands, the most commonly practiced tactic for holding an objective is to buy the smallest (and therefore cheapest) Firewarrior squad, buy a Devilfish troop transport that costs a third again as much, and just have them sit in it like a floating bunker all game. Aside from determining whether or not they're claiming an objective when the game ends, then, Firewarriors play no actual role in the battle. And that just seems silly.
It's not that they're bad troops. They have the best-ranged, strongest basic infantry weapon in the entire game, and with the assistance of a markerlight or two all of a sudden all those modestly impressive infantry weapons are hitting on 3's or even 2's. And as mentioned, they have a 4+ save, which is entirely respectable in this game for basic non-Marine infantry. But unfortunately, that's pretty much where their good qualities end. WS2, T3, I2 means they're pretty much guaranteed to be killed by anything that even looks like it wants to charge them, they're not especially good shots, they have the worst standard leadership it's possible to have in 40K, and their options are abysmal. They have two kinds of grenades, a markerlight for the squad leader and the option to trade their rifles for carbines, but the grenades are costly when they're not just downright counterproductive, the markerlight isn't networked and the carbines provide decidedly limited benefit given that Firewarriors on foot should be fairly terrified of being within 18" of an enemy unit. And that's it; no special or heavy weapons, no power weapons, no special rules, nothing but a very basic infantry unit that's frankly overpriced for what they provide.
But what would make them better? There are two basic options. The first is a cost drop. This doesn't make the individual Firewarrior better, but it gives them a certain utility of scale. Two squads of six are a nuisance at best, a non-event at worst, but two squads of ten or twelve? More bodies, and more importantly more guns, would go some way towards making the units in general more useful, because they could finally start to generate a meaningful number of shots. The second option is an expansion of the unit's current abilities. BS4, 15" rapid fire range and Assault 2 carbines have been floated before, as have giving Firewarriors access to decent weapon alternatives, like railrifles or heavy gun drones or networked markerlights. For myself, I favour the latter option. The Tau Empire's population is small, and they know it, so having them throw large numbers of disposable troops onto the field seems especially un-Tau-like. Having the Firewarriors who do fight have a better chance at doing it well, however, makes significantly more sense. And frankly, Codex: Tau Empire could use a bit of variety in its unit options; outside of the XV8 crisis suits, most units have maybe one or two options, many of which are total wastes.
The best thing that could be done for Firewarriors would be to allow them to bring some variety of weaponry to the field. It makes sense from a fluffy perspective, and it makes sense from a competitive, tabletop perspective.
And speaking of, I'd also like to take a moment to talk about the Devilfish, since it's almost exclusively used for Firewarriors. The Devilfish is not a terrible tank by any stretch of the imagination; it's got good front armour, and a few extremely good vehicle options, chief among them the almost-mandatory Disruption Pod. But like the troops it transports, it hasn't got much of anything in the way of weapon options, and that's a waste. As sub-optimal as Firewarriors are, a unit of them piling out of a Devilfish could still be a threat, provided they were suitable reinforced by said Devilfish. The last thing Firewarriors need are more S5 AP5 shots; why not mount a plasma rifle under the APCs nose, or a missile pod or two? Even Ork trukks come better equipped, with an identical gun with twice the range, and for a measly five points they can trade it in for a S8 AP3 rocket launcher. The way the Devilfish is currently equipped it just doesn't make any sense to field it forwards, which means that, like the Firewarriors hiding inside it, this unit ultimately ends up playing next to no role in the fight itself. And that's a waste.