I think all traders would agree that one's position size shoud reflect two things - the amount of risk one is prepared to tolerate, and the opportunity presented by the market in question. Let's assume we have decided our level of risk tolerance - the only thing then left to do is to evaluate the current risk/reward available in the trade.
If the trade is presenting a great risk reward, you should have a larger position. If the trade is good but not great, you would have an average size position. If the trade is acceptable but has some flaws (e.g. higher than normal uncertainty), then you would have a small position.
So, if we accept that position size should vary according to opportunity, "scaling out" not only makes sense, but is a requirement, in certain situations. If you have a position that started out great, but has now degraded somewhat, then you should reduce your position size. You may have started out long 20 lots, but now the risk has increased, the volatility is expanding, so the appropriate position size may only be 10 lots or even 5 lots. The expectation is still positive, but the risk is higher and/or the probability of success is lower and/or the reward has decreased.
Anyone who says position sizes should remain fixed is basically saying that all market opportunities have identical risk/reward profiles, and furthermore, that the risk/reward of a position does not change over its lifetime. I certainly don't agree with that. Look at any market and volatility fluctuates over time. If volatility increases, then other things being equal, you should reduce your position.
Thus - scaling out makes perfect sense in some cases.