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 invested on altcoin because I believe it is a good project and that it could mean something someday. 50% profit is too small if it will be big in the future. So I am holding.
But if it is just for trading, then 50% is enough profit. Dump and move on to the next investment.