I ask myself this question since I struggle to find a good answer. Sure, there are a few vaulted characters that everyone would like champed on their roster -Rhulk, Iceman, Peggy and maybe a handful of others. Other than that small number, what is the true value to having vaulted characters on your roster under this new environment? Here are my observations, but I would like to get the forum community take as well:
Pros:
1.) Boosted heroes - the biggest advantage that I can still put my thumb on is boosts for PvE/PvP. It can help having a boosted 4* (or 3*) and the more you have covered on the roster, the more boosted 4* you will get per event
2.) Team combos - You still get some advantage by being able to create more team combinations
3.) Variety - a small bonus, certainly little for actual game play, but it certainly is beneficial for some players to not use the same characters over and over
Cons:
1.) Power creep - so as new characters come out, they are, generally, more and more powerful/useful than the older vaulted characters - this is an issue since it basically reduces the value of 1&2 above significantly - while it may be good to have more boosted choices, does it matter if you are always going to pick the newest boosted characters who are the best anyways? I think this may be coming more and more the case.
2.) Champ rewards - vaulting seems to really have reduced the value of this feature since it is now very difficult to effectively champ and gain additional covers for 50% or more of the vaulted characters. This feature has certainly made vaulted characters less valuable than they were before.
3.) Frustration - more so for veteran players with vaulted characters but I imagine for newer players as well. If you are a veteran with a roster full of these characters who are not quite champed or barely champed, the frustration of knowing you will have a difficult time champing them or getting any decent champ rewards for them. If you are a new player, the frustration of knowing you will probably never get a champed Iceman or Rhulk etc.
4.) Required characters - whereas before in PvE, having a larger diversified roster gave you the advantage that you would have more of the required characters and perhaps have them covered better. Now it appears that only current token characters will be required characters for the most part in this new environment.