Presumably the deadbolt is far more secure than the door handle lock. One strategy a thief can use is to pick the handle while ignoring the deadbolt. This of course wouldn't gain them entry. Once the lock is picked they can remove the outer handle or lever which includes the lock cylinder. Now if you keyed your deadbolts and handles the same the thief has the information necessary to have keys to your place made.

You could have the deadbolts and door handles keyed differently so that the lock information the thief obtains is useless against the deadlock. Carrying two keys kind of sucks though.

You could spend more money to get better locks on the doorknob. But you'd have to spend a lot of money to be better off than having the deadbolt alone.

You could spend less money and get a door handle with no lock. If the handle has no lock the thief gains no keying information from it. They're forced to try the much more secure deadbolt to get in. For existing locks, you could pull all the pins and springs.