In my opinion, you can't compare raid bosses with challenge and adventure help. Challenge and adventure help isn't really "helping", it's just a way to gain a little extra XP. It's not so that a player can't finish his/her adventure or challenge if nobody helps. It's different with raid bosses. If nobody helps, you can't finish your boss. That's why I like the cap on adventures and the XP on challenges, to give everyone a fair chance to get some extra XP, and no cap on raid bosses to give everyone a fair chance to get them filled with other players and finish them.
