Litecoin is the second largest cryptocurrency available. Many markets have either adapted or are planning to adapt Litecoin. Adapting Litecoin on MtGox would allow persons to purchase Litecoin directly using USD or another currency. It will allow people to easier obtain Litecoins.
Litecoin has several advantages over Bitcoin. Litecoin generally has faster transaction times and is not anti-ASIC, leveling the playing field for miners.
The cost of implementing Litecoin on MtGox is minimal. It will increase traffic and will produce profits for MtGox.