I think you are confusing USB2.0 (USB second generation) verse USB1.0/1.1 with having 2 physical ports ... I ain't seen very many caddies that actually have 2 USB ports.
USB 2 (look for the words "high speed") allows a very fast data transfer between the computer and external device, round 480Mbps as compared with USB 1.0/1.1 at 12Mbps, just what you need for data transfer.
Look out for USB 3.0 it raises the data rate to 4.8 Gbps, a 10x increase.
I think you might have misunderstood. They have 1 port on the caddie but the came with a 'Y' cable so require 2 ports on a computer. The caddies for IDE seem to only require 1 USB port on a computer but SATA seems to require 2.
the reason for the second port is to provide the drive with power, 2.5" drives are aimed at being portable so having to carry around a power pack all the time and then having to find a plug isn't really practical if you're on the move
I think the reason they require 2 USB ports is because they need two lots of power (each plug can only draw 500mA). I've seen lots of 3.5" caddies that have two USB plugs, but I haven't seen any 2.5" caddies that can be powered from only one port.