Is it worth using? Riptides are obviously a strong unit, but their base cost is also very high. They're tough to kill, sure, but a Riptide's damage output isn't that great for how much you're investing.
A Riptide's real strength lies in it's flexibility. It functions as a Swiss army knife for the Tau army; properly equipped, a Riptide can handle literally any unit type in the game.
|It doesn't actually kneel to fire -|
he's just mocking the fire warriors
The reason ion/fusion is the most effective combo is because of what the Riptide is supposed to do. Yes, it can fill several roles in your army, but if you want to mow down infantry, why are you using a Riptide when 24 fire warriors will do the same job much better for the same cost? Or a unit of broadsides with missiles?
If you compare it to a more specialized unit in the Tau army, a Riptide will always lose out to missile broadsides, double plasma crisis, fire warriors, etc. It is not cost-efficient for its damage output. But despite this, Riptides are fantastic, because their job isn't to be a massive firebase like those units - Riptide's job in a Tau army is to provide stability.
While another unit might deal with one kind of target more efficiently, it's important to realize that an ion/fusion Riptide can deal with all of the targets you'll face, in every game you play. You can kill heavy infantry and light vehicles (firing normally); monstrous creatures (firing normally); heavy vehicles (nova-charging the fusion blaster or melee); normal infantry (ion large blast); and by taking a velocity tracker, it can also shut down flyers.
|Ion is the new hellfire. And kraken.|
And unlike most units of this variety (cough*sternguard*cough), they are incredibly difficult to kill. You can't just deal with a Riptide and move on to the next thing. With a 2+/5++ save, FnP from an etheral, and being able to nova-charge to a 3++, Riptides are the single most durable model in 40k.
This doesn't mean that every Tau army needs three Riptides. What's important to understand is that they can fill any gap in a Tau army nicely - a Riptide's ability to alter the specialization of its firepower each turn gives it unmatched in-game flexibility.