I've noticed that some places have a lock in the knob and a deadbolt in the exterior doors, and some places have a knob without a lock and just a deadbolt. Is there a benefit to having a lock in the knob I'm missing?
The lock in the knob can be set to be locked when the door is closed. In other words, you do not have to do anything extra to lock the door, just pull it closed. This can be good or bad depending on how you look at it (it can be easy to lock yourself out this way). The dead bolt on the other hand always requires an extra step to lock the door (put the key in, lock it, and remove the key); this forces you to have a key in hand, preventing you from locking yourself out.
Depending on what you are doing/replacing, also check your local building codes. Some types of deadbolts (the ones that require a key on both sides) may not be allowed in certain situations.
I agree with Jeff and Mike's answers, but you also have to make sure the lock was installed correctly. When I bought my house and changed the locks, I found out that the strike plates installed (deadbolt and doorknob) on the front door were only screwed into the trim with 1/4" screws. It wouldn't have taken much force at all to rip them right out. (My guess is one well-aimed kick.) The new Schlage locks that I bought came with 4" screws for mounting the deadbolt strike plate THROUGH the trim and into the studs behind the frame.
I completely agree that having more locks can be helpful but also keep in mind that the brand and model also play an important part. AFAIK, some locks (including deadbolts) are still susceptible to bump-key techniques.
Sometimes you really do get what you pay for. ;-)
It's quite useful to have two locks in the door separating the house/apartment from the surrounding world but use just one of them. Locks happen to break, although that's quite rare. If one of the locks starts malfunctioning you can just stop using it and use the other one until you fix the first one.
I once had such situation - I was trying to lock the apartment and noted that the lock mechanism was just twisted weirdly and the key would fit into it. No problem, I found the key to the other lock, locked the other lock and went to the shop to buy a new mechanism.
Most key-in-knob locksets have some vulnerabilities that make them better as convenience latches than as serious security locks. Adding a separate deadbolt is definitely worthwhile unless you are in a VERY low-crime area... and even then, I'd suggest doing it on the principle of "if you're just a bit more secure than your neighbors, the burglar will bother them instead."