For most fishing, especially rivers I would say 12 or 13 foot is best, but for still waters I think 10 - 12feet is fine.
The main reason for a 12 foot rod is the water depth, in as much as you want a rod longer than the depth you are fishing, not easy to fish a float in 10feet of water with a 10foot rod, unless you fish a sliding float.
Not many modern commercials are more than 10feet, hence the name of the rod, Shimano Forcemaster Commercial. The shorter rod is perfect for tight tree lined swims, for handling nice sized carp without being too long, and I would use it on most waters around now.