It can be useful to refresh your Deadly stack if you are instancing. Say you are up against a boss & you know he does some AoE fear/disorient, because the poison debuff only lasts 12seconds sometimes it is worth Shiving a refresher if it is about to run out & you don't want to lose your 5 stack.
Being an offhand attack it can be used if you are disarmed for some easy Combo points. But otherwise Eon is right. Its useful if you don't want to risk having your opponent escape from you. When I was old ShS I used to Premed>ShS>Garrote>Shiv-Crippling>Rupture when I was trying to bring down a warrior. If they can't handle 5-8 kiting you can just pop back into stealth, Cheapshot without DR, work your way into a Blind & repeat the whole process.
Its also good to Cheapshot>Shiv-Crippling onto mounted opponents in places like AV where 90% of the time they will just try to run away.