Sidechains allow a lot of really cool functions to be added to bitcoin, without having to actually change bitcoin.  Now the real bitcoin chain doesn't have to take a risk and implement a risky code.  After a long time if a code is proven it can be added, but for now the sidechains can be extensive testnets that are locked into bitcoin but not actually bitcoin. 

Sidechains aren't necessarily an altcoin killer, though they are a pump and dump killer.  If a novel alt coin is born, it can be locked into bitcoin and add value to the bitcoin network.  If it isn't, then it can be ignored. 

In many ways sidechains will separate the phonies from the true innovators.  It is needed. 

These guys making sidechains are extremely vested in bitcoin.  I am sure they all have huge stashes and are doing this to protect their investment. 

By allowing side chains into bitcoin, it makes it soooooo much difficult for an alt to gain a network effect to challenge Bitcoin.