There's two schools of thought on this.

Personally, I keep one copy of all rares, just in case I end up needing them. I also have all of my rare guardian hall slots filled. Do either of those end up providing me with any value? Honestly? Unlikely, but every so often I discover I have a need for a rare champ and I'm glad I end up having it, and not needing to hunt for it from ancient shards.

The other school of thought is to only keep the ones that have a specific use, and feed the rest.

Which you follow is up to you.

What are guardian hall slots?

The area that used to be training pit. In there is a section called Guardian Hall. You put dupe champs into it to boost your other champs from the same faction. Doesn't destroy them, but also doesn't remove them from your vault, so you need inventory space for them. You get various benefits to all champs of those factions, depending on how many slots you've filled.

Lol kram. How many rares do you actually use? I think the storage space is more useful than the guardian slots :) 

Probably around ... four. Lol.

But you'll never convince me to get rid of them, so don't bother trying :)

I feed pretty much every rare that I have with the following exceptions

• rares used for fusions (I still haven't fused Rhazin, as I frequently fuse his epics for tournament points)
• key rares that actually have use (the list is very very small.  It really boils down to just Coldheart, Apoc, Frozen Banshee, Gnarlhorn, Bellower and Reliquary Tender).
• rares for faction guardians for rares that are useful (e.g. enough dark elf rares so that I can make sure my Coldhearts have extra stats)

Everything else is treated as food.  The reality is that you get so many rares over the course of play that you will invariably have many copies coming in that if I were to ever need a specific one, then I'm sure I will get a copy of it soon.  This is especially true for non-void rares

i recently planned out all the rares i'll need for the game (ie: doom tower secret rooms), and put the rest on the chopping block. resources are better used elsehwere, including storage slots.  the rest won't ever be relevant, even if we hope that one day they will be... plarium isn't going to randomly buff a rare someday with lego-like abilities.

there's a few notable exceptions, but you probably already know them.  cold heart, fellhound, frozen banshee, starter champs, etc.

i didn't even bother with most of the rare faction guardians because there's so few i use. similiar to roseroyal's strat.

it really is this. I cannot be bothered to play vault simulator or pay for slots to buff anything other than a select few rares, and usually ones that are critical. 

Many rare champion is very good heroes what yyou can use specifis area. 

Starter characters, such as Kael or Coldheart (very good damage dealer at dungeons),Heiress (UK CB team hero),Reliquary Tender,Warmaiden,Soulbond Bowyer,Bellower,Metalshaper,Gnarlhorn (FW),Grappler (FW),Diabolist (FW), Fellhound (best xp farmer), Hellborn (World record CB hero),Frozen Banshee (CB poisoner), Pain Keeper (skill shortener), Paragorn (UK cheese),Renegade (skill shortener),.

This champions you realy need at end game too, if you didnt pulled out better heroes.

And other thing not just rare but some Uncommon hero you need to keep such as  Armiger, which is a god heroe in specific area.