No halo game that i know of has had a leaderboard. Sure you have been able to look at your stats and compare yourself with other players using external apps and websites but this is slow and requires you to stop what your doing, load up the app ect thus decreasing taking away from the game, also a lot of more casual players or beginners may not know about them.
I beleive Halo 5 needs a leaderboard that is not external and that you can access via the matchmaking menu.
1.It needs to be fast- if it is slow to load or laggy then this brings in the same problems as using an external leaderboard.
2.You need to be able to toggle gametype i.e Arena or Warzone.
3.You need to be able to toggle sub-gametype i.e CTF, BTB, Slayer ect.
4.You need to be able to toggle the map.
5.You need to be able to toggle variable of interest i.e total kills, Total assists, KDR, WLR, kills with weapon X ect.
6.You should be able to toggle who your compared with i.e the people directly ahead of you or behind you or to the leaders in that field .
Other games have done it very succesfully so there is no reason why halo cant.
Pros:
Promotes compeditive gameplay
Promotes goal setting/motivation
Alows you to compare yourself to others
Will help to keep people interested
Adds a leaderboards that you can view in game, removing the need to check your stats on a computer later or whatever you would currently do.
i.e Increase player retention time
Cons:
Development time + costs
People may feel self conscious when compared to others
Thanks for reading 
Note: I havent been around the forums for a long time so i am unsure if a topic on this has been posted before.