Board Game Arena

Surely this is a very odd feature to have?

By seeing all the cards someone has you can workout how many end game points they’ll get for their cards. In close games where you’re the last to play you can workout how many points you need to beat them.

In a real game you can’t look through an opponent's deck - what’s the justification for allowing it in the online version?

Well, all the cards someone buys where public information at some point. So you could take notes whenever someone purchases a card, and do the same thing you described.
So I don't see how that feature matters.

It is definitely different, as it takes away the element of memory that you would have in a normal game. I don't know of anyone who takes notes to know exactly what other players have but certainly someone with a photographic memory has an advantage.

As for this application, I don't think there is any other way they could have done it. As Manuel says, the information was public at some point so you do have the possibility of knowing exactly which cards are in which player's hand.