A question of Jockey wheels
Acording to wikipedia:
At which point do I need which amount of teeth in my rear mech?
Cage length, when combined with the pulley size, determines the capacity of a derailleur to take up chain slack.
Cage length determines the total capacity of the derailleur, that is the size difference between the largest and smallest chainrings, and the size difference between the largest and smallest sprockets on the cogset added together.
A larger sum requires a longer cage length.
Typical cross country mountain bikes with three front chainrings will use a long cage rear derailleur. A road bike with only two front chainrings and close ratio sprockets can operate with either a short or long cage derailleur, but will work better with a short cage.
Manufacturer stated derailleur capacities are as follows: Shimano long = 45T; medium = 33T SRAM long = 43T; medium = 37T; short = 30T
Long story short:
If you're having trouble using all the gears on your bike, and having trouble finding a longer cage deraileur, running bigger jockey wheels helps take up more chain slack and allows a bigger useable gear spread.
Otherwise you don't need them.