To surivive.
If you use throwing weapons instead of a crossbow and up to three quivers in a defensive siege, your fault. If you´re a dedicated archer you shouldn´t have a problem at all to loot enough spare arrows from your fellow dead archers or pick them off the castle wall/ground/siege tower (just like the odd throwing spear, bolt or axe). All you risk is beeing shot and that´s nothing unusual in that situation.
Bolts are far more tricky to obtain, as naturally, you want to use archers who fire noteably faster and kill at a much higher pace at point blank range whereas crossbows deal a lot more of punch at a steady pace but you´ll only use them if you play Empire.
Throwing weapons already have a very high ammunition capacity for their weight, just like average arrow quiver sizes are too high.
Again a cut of reality for the sake of gameplay and fun. Try to put 40 arrows in your quiver and see how you can handle that bundle.
A sheaf of english war arrows counted 24 shafts. Greek javelin throwers had around 3 throwing spears with them. So PoP´s already pretty high with it´s actual amount of ammuniton but since MnB lacks the mechanic of resupplying during a field battle it´s approbiate.
The infinite ammo for defenders is something TW invented, not PoP, just like they kept accuracy at 100% and PD making a damage bonus instead of a speed and accuracy one so complaining about it is pointless, you need to accept it and circumvent it smartly.
If you want to bypass that problem, especially with throwing weapons, esure to bring troops which throw stuff with you to loot ammunition from the fallen or use a crossbow.
And stay alive. I haven´t had many battles where I lived long enough to empty 3 large quivers of crossbow bolts. El Loco´s currently level 41 or 42, can´t recall it properly. And as he´s having around 400 crossbow proficiency he´s having a lot of fun together with the side-bayonet Mettenheim crossbow if the situation calls in for a sound and heavy thump.
One of the worst abusations of top-shotting was Hundred Years War for MnB - if you played english and you had taken care of the few french crossbowmen you simply camped on the ladder and either potshotted opposition or wrecked 250 zombies with your polearm, as overhead slashes have greater range than stabs, at least from my impression