As in all best practices it is advisable to diversify with any type of investment. So even with bitcoins and alt.
diversification means investing your money in safe assets to reduce the risk and maximize the profit chance. and these assets have to be somewhat not-connected.
altcoins don't match this and are not suitable for diversification.
when you invest in altcoins you not only increase your risk but also you reduce the chance of making profit. not to mention that they are connected to bitcoin price and each time bitcoin makes a big move they all go down.