Currently struggling with the same issue. 

I have a lot of level 50 rare characters that take on a niche role in dungeons and am not sure if it is worth levelling up rares to 60.

Epics and Legendaries do seem to have higher stats.

Based on my first impressions especially attack rares struggle in dungeons past level 13.

From the rare characters I levelled up Athel, Warmaiden, Coldheart.

Warmaiden and Coldheart keep dying in Minotaur level 13, 14.

Is it necessary to use Lifesteal set with all rare attackers to keep using them in higher dungeons?

I think about levelling up Apothecary which some say is one of the best rares.

But ever there I wonder if I should rather wait until I get an epic healer who can perform a similar role.

I liked using Conquerer for Ice Golem because he stops the respawn.

But will he survive in dungeons higher than level 13?

- - -

Until faction war is released I wait with deleting my rare collection.

But even then I assume in the long run it is necessary to have epic and legendary characters for each specific niche role.

It would be great having some feedback from players who clear dungeon levels 14-20 if rare champions still can be used.

Well, the problem I'm dealing with is a cap in the collection and the vault.  I can't keep them all.  So what I did was eat all the duplicates for now and I have a little over 100 unique rare.  That will work for now.  I figured I have no idea which will be needed in future fusions so I would like to keep at least one of each and that seems to be working for now.

Also keeping at least one of each rare. For rares that are useful I use the duplicates to push that one in skills, so I can use books on potential new heroes in "need"

Kael ---> Spider and Clan Boss. Poison effect and fully refill turn meter when kill spiders. 

Athel ---> Fire Knight. Default skill hit 3 times. Good to break Fire Knight' Armor.

Elhain ---> Spider and Campaign.