Early on in accounts it is far easier to book rares. Books can make a big difference in the champions performance.

Later on certain rares can be used in very specific comps and for things like speed runs. I still use renegade in many teams and for speed clearing DTH floors.

Coldheart is also an exceptional rare who Ive full built three of so far.

You'll almost never see one in higher tiers of arena, but many of my teams have a rare who happens to just have the right skills for the job.

Heiress for example, is used as a cleanser on a lot of demon lord teams.

Bellower in a stun set is great for cc even in the upper levels of doom tower and sin city.

I use Armigar on my Dark Fae hard 120 team, and he's an uncommon. I could probably swap him out for someone who does what he does and more, but ain't broke, don't fix.

It's a few things. First, Rares on average on the fewest amount of abilities like Passives, some don't even have a A3. Second, they're just not powerful with good base stats or having the best versions of buffs/debuffs.

Lastly, for Arena especially, there's not really any rares that do well in arena. Paragon is a great "Troll" rare for Arena, you may see him in Solo Defense teams. There are a couple others but usually people have a Epic or Leggo that is better overall. Outside of Arena there are a few awesome Rares.

Best is Coldheart, hands down no question. Then you've got Bellower, Apothecary, Reliquary Tender Kurzad ect. Painkeeper is another one that has a niche in Unkillable Teams