Did the average barbarian warrior have a missile weapon alongside his sword and shield?
Yes, both in the pre-Roman and the Roman Iron Age Germanic warriors generally carried one or two throwing javelins besides a broader stabbing spear. We find them less often in burials, possibly because they are less prestigious/symbolic than the 'main weapon' spear, and possibly because they often had little metal parts (either bone or simply sharpened wood point). Still, we have them from descriptions like Tacitus' Germania and Annals, and from weapon deposits like Hjortspring and Illerup Ă…dal.
By the fourth century A.D., aside from javelins, many Roman infantrymen carried a number of short darts known as plumbatae or martiobarbuli. Made of lead and with a barbed tip, they were thrown by the first few ranks of soldiers prior to the main infantry clash. They were likely attached to the interior of the shield above the central grip. As with javelins, they were not intended to be decisive, but rather to disrupt enemy formations prior to the two lines closing.
The Roman Javelin (the pila) was quite effective by the standards of the ancient world. What made the Roman pilum so effective was the fact that it could not be used against them after they were thrown. Its iron head was attached to a wooden pole by two metal rivets. One of these was replaced by a wooden dowel, so that the head was bent or snapped off entirely when the pilum reached its target or fell to the ground.