Why does the Covenant use Grunts as cannon fodder?

After seeing how seemingly cowardly Grunts are and after learning that they need to breathe methane, I've always questioned why the Covenant would use them so extensively as cannon fodder.

I've looked around some for an answer but haven't found one that satisfied me. The stated reason is that they breed quickly, lend naturally to group tactics, and don't have the build for many other combat roles (plus their cowardice mostly stems from lack of training). However, this doesn't address the fact that they must breathe methane and thus every Grunt needs a breathing apparatus to live.

Sure, this is the future and probably have the tech to not have the apparatus basically be a space suit, but it is still a very expensive piece of kit for an expendable unit. This isn't to mention the safety issues from managing large quantities of explosive gas in a warzone, especially when a significant number of units are wearing tanks of aforementioned explosive gas.

I have a hard time believing that it's practical to equip such large numbers of Grunts when the material and logistical costs are so high.

Any thoughts on this?