If the price goes up, you can buy back without a loss -> loss in BTC count yes, but no loss in fiat. You just missed 5-10% profit!
The value of fiat diminishes and bitcoins value grows. So in the long run a bitcoin loss is more substantial that a fiat loss.
If bitcoin value grows
It's just plain and simple risk management.
Of course you can tell everybody bitcoin will reach 1 Million USD per Coin and selling is allways stupid -> but to sustain such levels we need much more users and money! We got rid of all chinese buyers! That's really bad!