Let's say I am spending 250 USD to mine a coin of worth 330 USD, selling it immediately doesn't give me a big profit. If I am selling the same coin for 500 USD then thats a good strategy.
(amount spent on mining per coin in FIAT) : (Value of the coin)
when you get a good ratio convert the coin to FIAT.
Actually most of mining sites require an initial deposit for you to deal or to mine a specific coin. And we could hide the fact the if we are going to choose to sell it early , could not earn more or we could also loss a part of our initial deposit. So the best thing to consider to do is to save at first and then look at a way for you to actually deal with long term investments so you could have a profit with the used of the coins you've mine.
It is probably silly like you said but not silly if you have some saved in your wallet considering the value increase over time. Who does not want some extra cash in the next few years from their saving.
Most especially a huge percentage that no bank can ever give or offer you. However, if you know you have some extra cash to always play with without having to touch your bitcoin, I see no reason why it should be silly to leave your bitcoins intact in your wallet.