There's nothing awe-inspiring about Piranhas - they don't offer massive damage, and they're fairly easy to kill if your opponent decides to focus them. But they fill some important gaps in a Tau army, and you can use them in a variety of different ways, which is what makes Piranhas such a strong choice in almost any Tau composition. It's also worth noting that Piranha are incredibly cheap for what they do - if you subtract the cost of their drones (14 for drone units), Piranhas cost just 12 points each.
Piranhas excel at a few specific roles: Hunting down hard-to-reach units, Tank Killing, and Screening other units in your army from assault.
|It's hard to find a greater good|
than 12 point skimmers
Hunting: Piranhas are fast skimmers, which lets the move 12" and fire at full BS. Gun drones now count as passengers, which means that they can only fire snap shots if the Piranha moves more than 6". If you flat out with a Piranha, you can move an additional 18" in the shooting phase instead of firing for a total distance of 30".
Because of this immense mobility, Piranhas are great at cleaning up lightly-armored enemy units that the rest of your army may have difficulty getting to. Small units of cultists, guardsmen, gaunts, or other inexpensive troops are useful for holding objectives in your enemy's deployment zone, especially when they've set that objective behind a building or other terrain that blocks your LoS. Deploying 12" forward and jumping 30" on the first turn means your Piranha can hit anywhere on the board come turn two with a barrage of S5 AP5 weaponry - exactly what you want to chew up cheap objective holders.
Even if your enemy decides to counter this move, a unit of Piranhas is surprisingly durable. Front armor 11 makes them resilient or immune to most infantry weapons, and their 4+ cover save from moving flat out will let them soak up a lot of fire before going down - probably more than your enemy is willing to commit to put down a full unit. Better still, you can detach your gun drones on the second turn to force him to deal with an additional target roaming around his backfield.
|S8 AP1 vs unsuspecting cows|
Piranhas are especially talented at downing vehicles with the fusion blaster's new 18" range. Because you can move 12" and fire at full BS, and get 2d6 for armor penetration rolls within 9", you now have an effective threat range of 21" for taking down heavy vehicles. Even the base damage of a fusion blaster is solid for taking down smaller tanks, especially due to the AP1, and they can serve as decent monstrous creature hunters against vehicle-light armies.
Screening: This is the most important use for Piranhas because it mitigates the Tau army's glaring weakness: close combat. Against some opponents you'll have time to deal with the terminators or plague marines slogging their way towards your lines, but plenty of lists can shove three or more major threats in your face on their first turn, and you might only have the firepower to eliminate one of them.
A well placed Piranha unit can negate this problem, at least for a turn or two. Leaving your army to deal with the most important target, you can spread a squad of Piranha out just in front of your army to screen your fragile fire warriors and battle suits from enemy attack. A Piranha has a wingspan of about 6". Since enemy models need to stay at least 1" away from your Piranhas (unless they're assaulting the unit), you can create a 40" wall in front of your army by carefully spreading out a full unit - 6" per vehicle, plus an additional 1" on each side of the wings.
|"Great Wall of Skimmers" is a little less catchy|
You probably won't need a full 40" screen very often, though it's a nice option to have. What's important, though, is that you only lose 10" of your screen for every Piranha we remove from the unit, so you can use this tactic very effectively with just a unit of three to provide a 20" screen.
It's important to be careful of an opponent destroying one or two Piranhas and charging through the breach, but this is where the gun drones come in. By detaching the drones before your skimmers move and using their thrust move to re-position, you can set up the drone squadron as a fail-safe screen. It won't block off as wide of an area as the Piranha but you can make sure a vulnerable section of your line is doubly safe against assault.
Are you going to lose your Piranhas with this tactic? Almost certainly. Your opponent is either going to go around you, create a gap, or absolutely trash the Piranhas that are in his way. But the screen should be able to cover your entire army if you set it up right, and that means you'll be able to unload a monstrous amount of overwatch into whatever decides to charge the squadron And whatever's left over is sitting right in front of your army for fire warriors to triple rapid fire it into a bloody smear.