Jet engines are not very efficient at low power and speeds, so using an alternative method of propulsion is a good opportunity to save fuel.
An aircraft the size of an A320 will probably burn about 200 kg of fuel during taxi (based on this and this). Assuming fuel costs \$3 per gallon, this is about \$200 per flight.
The cost of the added weight from the electric motor will vary based on flight length, but assuming \$0.05 per pound, the extra cost would be \$16.50. Of course you are saving 200 kg of fuel, so this also helps to offset the weight. The electric motor is powered by the APU which uses much less fuel than the engines.
As KeithS points out, this adds up quickly. Even if United Airlines only equips its 150 A320 series planes with this equipment (which would certainly be a large upfront cost), and those planes make 5 flights each per day, that adds up to \$150,000 in savings every day. This will of course depend on fuel prices, which are actually well under $3/gal at the moment.
A simple way to save some fuel while taxiing is to only use 1 engine (or 2, for four engine planes) to taxi. This doesn't save as much fuel as the electric option, but also has much less upfront cost.