All glaives take 4 shots to charge, even the exotic ones. As far as the exotic ones in particular go, the biggest problem isn't ammo capacity or reserves, but that their perks are tied to having a [u]full[/u] shield bar, and using the shield for any reason immediately removes your ability to use the exotic perk until you fully charge it again. Personally, I think the best ways to make the exotic glaives more useful is removing the need to have a full shield charge to use the perk while making the perk last longer/ get stronger the more shield you have, or completely separating the exotic perk from the shield bar.