I try to cover my bases, on anything that might be valuable in the future, so I purchase alt-coins and hold them like I would bitcoin.  I usually just look at the total market cap of the coin and whether it offers something new, or has potential.

I have at least a millionth and sometimes much more of Ripple, Ethereum, Dash, Bitshares, Maidsafe, Next, Peercoin, Monero and Counterparty.

I also have some exposure to Litecoin, Dogecoin and Stellar, but don't see much point to them beyond network effect.