Yes, they're fragile, and yes, they are very weak in melee. Fire Warriors also have a low base Ld. They are not a well rounded, versatile unit like a Tactical Squad. These weaknesses are irrelevant though, because Fire Warriors have exactly the right attributes to serve their role in a Tau army.
A Fire Warrrior's only strength is shooting, but that's exactly what we want. Because of this narrow focus on one style of combat, Fire Warriors damage output and durability is fantastic for such a low-cost unit.
|He's like Clint Eastwood with hooves|
Their 4+ armor save is just high enough to make them resistant to anti-infantry fire, which is all that they need. An AP2 or 3 weapon will kill a marine just as easily, and most AP4 weapons - assault cannons, autocannons - are intended for killing high toughness targets or light vehicles. Killing Fire Warriors with S7 weapons is a waste, but if an opponent does decide to focus a unit with low-AP weaponry, they are inexpensive enough that it's usually worth going to ground.
The S5 of pulse rifles is a big part of this effectiveness. It lets them cut down low toughness targets easily, but more importantly it also makes Fire Warriors a threat against transports and monstrous creatures. A T6 Bloodthirster makes bolters worthless, but not pulse rifles.
But more important than anything else: pulse rifles can rapid fire at 15". This is an enormous advantage for the Tau because it allows you to unload heavy fire into an infantry unit while being very difficult to assault. Even being 14" away isn't a problem, because you'll be killing some or all of the targets within rapid fire range with your shooting, leaving the enemy unit 9 or 10" away from you.
|Pulse rifles don't care about |
your storm shield
Granted, this tactic won't work against bikes, cavalry, beasts, chariots, or jump infantry - 15" rapid fire isn't foolproof. You'll be able to focus down some of these units before they reach your army, but some will inevitably make it through and crash into your Fire Warriors. But that's okay too, because despite the potency of their firepower, a full unit of Fire Warriors costs slightly more than 100 points - they're not a high value target, even if they hit like one.
Other units can put out similar or superior amounts of firepower, but none so inexpensively. And more importantly, none of these heavy fire units are Troops. Fire Warriors don't take valuable composition slots away from the rest of your army, a problem that forces some some codexes to take undesirable Troop choices just to hold objectives. But a Tau player can fill all of his army's anti-infantry needs and simultaneously give himself several scoring units.
You'd be hard pressed to build a bad Tau army by starting with six units of twelve Fire Warriors, because they're just that damn good.