In the Israeli version of Rummikub, the joker is used far differently than that of the American version. If everyone plays Open Rummy, the jokers then become more or less common tiles since they can be replaced and used in other melds. They are not as versatile as they are in the American version, even under these circumstances, since they can be used only to make new melds instead of being rearranged. They can, of course, be layed off on runs or groups of three, but here they serve only the advantage of lowering your rack by one tile. If this is the tile that puts you out then that is great. Otherwise it would be a basically worthless move.
On your rack the joker is still the most versatile tile you can hold. The player having one joker has a considerable advantage over those with none when everyone is on the rack. A player with two jokers is in a very strong comparative situation. The joker becomes basically a tile that can take any position needed. Therefore if you are using it to form a run or group, and you pick the same tile, you have actually picked the equivalent of two tiles at once since the joker can now be used to fill any other combination that you are holding.
The other advantage of a joker is that it is the highest scoring tile. In the Israeli version of Rummikub, 50 points are needed to meld on the table. The joker is often the key to being able to make an initial meld. It is advantageous to open without using one because the joker then loses its versatility on your rack and becomes available for other players to use. They cannot use it to make their own initial meld, but once they have melded, it is no longer your exclusive property.
In any even, as in the American game, if you play a joke in an open meld, try to place it so that it cannot be reused. If both of the tiles which the joker represents are already tied up in sets or in the discard piles, the joker is absolutely safe on the table and at that point counts for no more value than any other tile.