It's almost inevitable you'll drop your iPhone, and if it's caseless, it'll break both your device and your heart. But with Systm's cases, you might ensure that your phone will have a soft, safe landing.
Now if there's any chance you might upgrade when the new iPhone comes out (which is probably soon!), you maybe shouldn't buy a case now. But if you're sticking with the 4S, these rugged sleeves ($25-$50) will protect your Apple gadget with reinforced bumpers, cushy, corners, and Poron XRD foam in the pricier versions. But better to spend a little money on a case than a lot of money on a new phone, more here.