To be a great player, you need to be able to play any position for the sport you're playing. You may have a dominant position that you always play, but if someone gets injured or just to change things up, you need to be ready to play any position. Some teams learn each others positions while others don't. However, I do think it would be helpful to a team in the future if all players could play all positions.
When it comes to volleyball, there really aren't that many positions. You have the setter, hitter, passer and then if you count serving, there are only a total of 4 different things you have to learn. Now that isn't that bad, especially when compared to something like baseball or softball. One of the most important things you should be able to do over anything in volleyball is serving. If you can't serve, you're team is in trouble because your team isn't getting points! Some coaches even go as far out as buying volleyball equipment for serving targets so their team can learn how to serve in different zones. However, I personally wouldn't buy equipment for this, but just put a chair, trashcan or even the coach in the spot where the players should be serving at.
You could probably come up without a lot of creative volleyball drills for serving, so just think real hard and I bet you will! Also, when buying wholesale volleyball equipment, make sure you need a lot of it first. I know it's probably cheaper ordering it this way, but say you only need 1 of this item and it comes in packs of 25, what are you going to do with the other 24. Well just think about and I know you'll make the right decision!