Your altcoin goes up 50%. Options:
1. Hold. You think your alt will beat BTC returns.
2. Sell initial investment to BTC, leave rest in alt. You think BTC will beat your alt (or you are unsure).
3. Sell initial investment to USD, rest in alt. You think BTC and alt will go down....??
I never suppose for so high growing rates. As for me it is better to get guaranteed profit less than 20% than take a risk. I hold my savings in some well-known currencies and I am dont worried at all.