The problem is that giving it a value will not solve the problem of strange trades, no matter what the value is. Basically the Joker affects the value of other pieces. E.g. if both sides have a Queen and some Pawns, but one side a Joker (and one less Pawn) the side with the Joker probably has the advantage. At least, I would not trade it at the earliest opportunity for a Pawn. But trading Queens would make the Joker much less useful, and the player with the Joker should avoid it. But to do that it would have to understand that the Queens are not equally valuable. Otherwise it would happily trade them no matter what value was given to the Joker.
The problem is that giving it a value will not solve the problem of strange trades, no matter what the value is. Basically the Joker affects the value of other pieces. E.g. if both sides have a Queen and some Pawns, but one side a Joker (and one less Pawn) the side with the Joker probably has the advantage. At least, I would not trade it at the earliest opportunity for a Pawn. But trading Queens would make the Joker much less useful, and the player with the Joker should avoid it. But to do that it would have to understand that the Queens are not equally valuable. Otherwise it would happily trade them no matter what value was given to the Joker.