It depends entirely on the game and status in question, obviously. Some are useless, others are game-breakers.
A sleep spell with 100% accuracy is the perfect set-up for the low-accuracy nuke attack. On the other hand, you get sleep spells with maybe 50% accuracy that sometimes allow your opponent to wake up the very next turn. Those are the useless ones. Status-inducing spells have their place, and can be extremely useful in a pinch.
Generally, the longer it takes to defeat enemies, the more useful status ailments become. Poisoning the enemy for even 10% of their HP each turn doesn't seem like much in a game where one round of attacks will win, but in games where every battle takes a considerable amount of time, that 10% starts to look mighty impressive. It can even lead to status-ONLY builds where you essentially immobilise and poison the enemy, then stand back and laugh, renewing either as necessary.
Status spells can be incredibly useful against bosses, who generally have many many hit points, but ironically also have immunities to various statuses most of the time. Here's an example: some of the bosses in Earthbound can be paralysed, which is permanent immobilisation. By the end of the game, most enemies can be killed long before a spell like that pays off. Early on, it's a boss-killer.
It should also be noted that I used statuses extensively in FF: Tactics Advance, and they were damn powerful. There is nothing like beserking the White Mage into meleeing your Fighter with Strikeback (ie. the White Mage will attack the Fighter, who will block and counterattack.)
