I think considering your piranhas 'escorts' for your troop transports is great from a fluff or storytelling perspective, but on the table, they are a separate unit with very different abilities. For starters, Piranhas are FAST, and able to zip from one side of the battlefield to the other to contest some vital objective. If you have piranhas, you must be willing to use them for what they are good for, even if that means 'breaking formation'.
In the escort role, the only thing that the Piranhas can really provide is cover, either from shooting or charging. Piranhas are big for their point cost and, depending on terrain, could reasonably give your transports or disembarked troops a cover save. With their mobility, they can always interpose themselves between your valuables and the most likely danger. However, depending on the Piranha flyer base height and the size of your enemies, they might be able to shoot over or under them without cover, and more mobile enemies can reposition as well to get a better angle.
They might be more useful to block an enemy charge, though. Your unloaded troops and transports might be threatened by fast assault troops (won’t stop jump troops, obviously) or perhaps flamer-armed enemies. Drop your drones and then just interpose your piranhas. Put them right in his face, so he can't even move properly. He can blow them in the shooting phase, and charge through the craters - but if he is blocked in the movement phase, he should not be able to make the charge anyway. And Piranhas are very broad, so two should really ruin his movement corridor. It's a sacrifice, of course, but depending on how you value the drones - which you should deploy before sacrificing - the chassis itself is only ~20 points. And the enemy still has to devote firepower to destroy them – or charge and risk getting caught in their explosion.
At this point, this is turning more into general advice about Piranhas, and my own experience with and my favourite uses for them:
First off all, Piranhas are NOT fragile and are actually decent at soaking fire
In fact, they are surprisingly resilient – for their price. Don’t expect any one piranha to live, that your enemy really wants dead, of course. But taking them down protects the rest of your army. Yes, they can be damaged by bolter fire as people like to point out, but only from the rear. And even supposing the enemy gets to take rear shots at your piranhas, you will actually be okay. It takes two glancing hits to down a Piranha, so that is twelve bolter hits from the back – or twelve heavy bolter hits from the front. Twelve bolter hits on a battlesuit will inflict two wounds on average killing one suit. But a suit is rather more expensive than the ~20pts Piranha. There are weapons that are more efficient at taking out Piranhas, but your enemy will still be hard-pressed to justify the expense of firing more valuable weapons at your cheap skimmers. Especially because you can opt to Jink them to get cover even when you are in the open (where your Piranhas honestly will often find themselves, because you will need to reserve good cover for more deserving units).
Lone Piranhas or small squadrons can effectively perform these manoeuvres:
As others have pointed out, piranhas are great at threatening vehicles. A fusion piranha is ~30 points sans drones, which is not the best firepower for the price. However, it has okay survivability and great mobility. The presence of one or two (deploy them singly if you can) of fusion piranhas helps to keep your enemy worried and force him to manoeuvre his tanks more defensively – or hunt down your piranhas. For the actual attack, you should use markerlight support to make up for the rather poor BS.
(Be sacrificed to block)
Is there an enemy unit you just can’t allow to approach this turn? Like a land raider full of assault termis or that big wave of orkyness? Just land your paper-thin little skimmer right in front of it. It will be blown to bits in the shooting phase, but there isn’t much he can do about it in the movement phase, except tank shock it – which only clears the way if he manages to explode your piranha, which is highly unlikely.
Usually, denying the move will be enough to prevent the assault. And even a destroyed Piranha will force move through cover rolls. Remember that delaying the enemy charge just one more turn is a huge strategic goal for most Tau armies.
Carry the torch
(Move Seekers behind the enemy)
Some vehicles like imperial tanks are very tough in front, but weak in the side or back. Provided there is good cover on the side, you can rush a piranha forward with its blazing speed in one turn and in the next put two rather big holes in the back of some tank with markerlighted seekers. Later, you can use the lone piranha behind enemy lines to contest objectives, continue to threaten enemy tank rears with the burst cannon or lock enemy heavy weapon squads in melee with the drones.
Big swarms of Piranhas can:
(Use their size)
Piranhas are HUGE for their point cost. They can really cover the board if you bring them in numbers. On a confined map, this will stifle their mobility, block the rest of your army and make them fall prey to template weapons. But on an open field, especially if you are worried about enemy deep strikes or other surprises into your lines, you can just spread them about as you would kroot, to make deep striking close to your more valuable units more difficult and dangerous.
(Form a mighty, highly markerlight efficient, shooty unit)
Take a maxed out swarm of five Piranhas for 200pts. This gives you 40 BS3 /BS2Twin pulse shots. Add just two markerlights (attached drones profit from the markerlights of their parent vehicle), and this becomes an average of more than SEVEN dead marines or similar. Add two more markerlights for scour, and you will absolutely slaughter imperial guard or orcs. The only issue is range, as the piranhas are so bulky that not all of them might be in range. But remember that vehicles in squadrons don’t cover line of sight for their own squadmates shooting.
Macross Missile Massacre
(Unleash all the seekers at once)
Not necessarily the best value for money manoeuvre, but it’s hilarious and often makes your enemy’s chin drop. Build a full squadron as above, but add the maximum of ten seekers for 80pts. Seekers can be fired as ‘seeking’ on markerlights, but they can also fire as regular missile launchers, which is far more markerlight efficient. Just light a big, dangerous enemy unit or vehicle up as above, and fire 10 S8 AP3 shots at 72” range with BS5 and ignore cover. This will kill many monstrous creatures, tanks and vehicle squadrons outright. It’s murderous to Tau battlesuits, too.