Not too rarerly, though I don't mean every 2nd or 3rd hand, a defender will find himself in a position where there is only one suit where he can get tricks -- or in a trump contract, only one side suit where he can get tricks. If there are trump tricks to be hand, they will wait, but side suits won't necessarily do so. When you find yourself in that situation, and declarer is setting a suit up for sluffs, you'd better go for that only suit where you might pick up a trick or two, if on lead, regardless of tenaces. After all, your partner's hand might fill in the holes in your holding, and anyway, provided declarer will evidently have the winners for sluffs, that trick's a goner anyway on a "safe" lead.
Above are links to some illustrations.