Which player made the choice not to have a skill (like Arcana)? You keep saying "they", as if "they" were a person, making choices.
All of them. I'll give you an example:
In a recent campaign I started playing in we all came up with characters independently. I play defenders mostly when I play but really wanted to try an Avenger. Well, we came to the table with four strikers and two leaders.
We hashed out roles to cover more bases AS A PARTY.
Two of us became defenders for party composition. We didn't have an INT-based character and we had two Avengers so I switched to a Deva Swordmage vs. the Deva Avenger (still using my Asura mini

Now we don't have the Arcana weakness, some area blast/bursts for minion clearing and we have a pair of defenders in our six-person PC group. Am I playing what I initially wanted? No, but the game is fun because I cover roles we wouldn't otherwise have covered and making things more difficult.
In another party, Arcana could be covered just by someone having a minimum 13 in INT and Multi-Classing to Wizard. I could have done this with my Avenger had we not also been short defenders.