A lot depends upon how deep they are as deep ones can sometimes beat the objective you are trying to reach.
From memory I think there are around six or seven holes in them and the temptation is to use all six or seven, I would suggest three at the most.
What happens is if you use more you crowd most of them out so you only get the effect of around three good plants.
The other thing is, is watering -- the top area can get quite dry and the bottom quite sodden so increases the weight as Jaybe rightly mentions.
I would suggest one plant on the top,one about a third of the way down and the third about half way down, place these two on opposite sides of the bag.
Then with a bit of luck you will get a well shrouded bag and hopefully lots of berries.
I hope that helps but I have to say I don't like them but if needs must then yes I would use them.