There's another possibility that may explain why Goliath only went into battle with David when his armor-bearer went with him: perhaps the two were friends.
When we're talking about armies and battles and wars, we cannot let ourselves think that there's just a corral full of armor-bearers hanging around in a certain part of the camp and when it's time to engage the enemy, you just pick one and go. Or that one just steps forward because it's his turn. Rather, armor-bearers were generally assigned to warriors; they became a pair. Wherever the warrior went, the same armor-bearer went with him.
And we can assume that this was true in Israel because not once do the Hebrew Scriptures ever tell us that someone stepped forward with 'an' armor-bearer, but always with 'his' armor-bearer. Jonathan famously crossed the breach with his. Saul asked his armor-bearer to kill him when he was gravely wounded by enemy archers. There comes to be a strong relationship between these men, maybe even a friendship.
In modern terms, we could talk maybe about military bands of brothers who go into war together, and that's close. But perhaps what is closer is the chaplain and the chaplain's assistant. The chaplain assistant's entire job in the military is to be glued close to the chaplain and to provide protection for him or her. It is the chaplain's assistant who carries the weapons, the chaplain's assistant who strikes down an approaching enemy, the chaplain's assistant who ensures the safety of the chaplain while he or she carries out the chaplain's duties. There comes, then, to be an incredibly strong bond between a chaplain and a chaplain's assistant, a bond in which the chaplain's assistant comes to share the burden of the chaplain's work and to take a real ownership in what the chaplain is able to do under his or her shield.
This is the kind of relationship that I imagine with Goliath and his armor-bearer (or really, any ancient soldier and his armor-bearer). This armor-bearer has probably been with Goliath for a long time. He has probably come to take some measure of ownership in the giant's victories. He has been there through thick and thin. If you imagine an ancient version of a foxhole, these two guys are sharing one. Saul had a protector who slept next to his head in the fields; this is the same thing. These guys are brothers, and there is a certain love between them.
So to think that when Goliath steps forward, he's going without his brother would be ludicrous. To think that Goliath is going to go into battle and not take his friend is crazy. This guy is a huge part of Goliath's battle plan. He's a big part of Goliath's successes. Even if Goliath is a great warrior and a big man and heavily armored, there's something about having a friend with you that just changes the whole dynamic. It changes everything.
And you don't take a guy whose job it is to give you his life, who has been there by your side in every ditch and battlefield for maybe your entire fighting career, and tell him he can't take part in your greatest victory. And tell him you don't need him this time. Not even when the enemy appears to be nothing more than a little boy with sticks and stones (which may, we must point out, break your bones).
If this is true, then Goliath is no scaredy-cat stepping forward with his armor-bearer; he is a faithful friend, one ready to share his victories even when it seems he may not 'need' to. That's important, and it's beautiful.