For the same reasons you'd want to hold both gold and silver, the same could be said for holding Bitcoin and Litecoin. I really wish more Bitcoiners would keep an open mind on Litecoin. Maybe I have bad luck, but I tend to run into people online that believe there should only be one blockchain for the world and altcoins are either useless or pump/dumps.
That's not fair! Hopefully, they change their attitude with time.
Your argument is contradictory. You recommend people hold only 2 currencies and at the same time express your love for the world of altcoins! I see absolutely no reason to invest in LTC at this price point. Seek out the best names and best developers, that's where you'll find tomorrow's winners.
Right now, I see Bitcoin and Litecoin with the most foundation (BTC obviously 10x more). I'm not against altcoins at all, but I'm not confident in which ones will end up succeeding. I personally believe we will have 4-5 successful coins universally accepted around the world. OR hell, we might even end up with universal wallets and a merchant can just say they accept crypto and whatever coin you use is auto converted to what they want to hold. The future is uncertain.
Looking at all the comments I'm still not sure why so many Bitcoiners hate Litecoin or think it's dead or had no value... whether they like it or not the halving event comes up on August 25th and there will be panic buying a week out, guaranteed. In 2016, Bitcoin will probably see the same thing with their halving event. The crypto communites should all love one another and stop the infighting!