Although it would technically be realistic from a certain perspective, it really wouldn't add much besides a tiny bit of "bean counting". Wear and tear is spread out over so long a time as to be not worth the trouble, IMO. As a roaming army/warband and villages handy every little bit, visiting a blacksmith wouldn't be difficult.
On the other hand the concept that a weapon might break on occasion in battle (rendering it useless for the remainder of the battle, but repaired afterward automatically) would be interesting. Force you to bring a spare just in case that 1 in a 1,000,000 chance of your sword breaking came on. Could have variable based on the weapon. Swords being more likely to break when beating on a heavily armored knight, but maces/blunt weapons rarely if ever, weapon material/quality affecting chances of a break, etc.
Would also give a reason to have to go back to your "supply train" or "squire" and pick up a spare or something. Extra weapons could be carried on your horse... A lot of ways this could go.
