Microtransactions are a fact of life now, something that's inevitable in any online multiplayer experience. Of course there's a right way and a wrong way to implement them, and in this case, the $25 spent within Halo 5 is actually the right way; for optional items only.
The new singular Arena REQ bundle contains a whole host of different armor, weapon and other skins to help you define your individuality within the Halo universe. You don't want to be just any ol' Spartan, now do you? But this is a limited offer with 14 separate packs smashed together that have "Mythic" and "Legendary" items that you likely won't find elsewhere. It'll be available starting February 16th.
But, even though it seems like a lot of money, it's cosmetic only and doesn't change the gameplay or give you an advantage. Aside from people staring at your newly applied weapon, armor and other skins. So don't fret, because you don't have to buy it.