The Halo sandbox has always been about empowering freedom, and to a certain extent, classes allow you to choose your style of play. But the beauty of Halo and the freedom it provides, is that the game allows you to experience everything in the sandbox everytime you spawn and only limit itself with the map that is played on.
Hey! Do you want to be a sniper?? I saw a sniper rifle laying on the back of the map, and further down lies an active camo that you could use.
Hey? Want to party up in a warthog? Grab your friends and arm up with plasma pistols to hijack the incoming enemy hog down at mid!
Hey! Want to be a tank assaulting the fronts? Go ahead, take the overshield below and grap yourself a saw or a chaingun turret and go ham!
To restrict gameplay elements for the sake of diversity and team dynamics is bad for halo, because that the structure that Halo games have do not gain from having such a system. Halo is the only game that can bring so many types of playstyles by having the sandbox in the playground, rather than selecting it in a menu. And with the HORRIBLE ui H5:G has, it just reinforces the fact that classes would not fit in a halo game. Hope I made some sense.