That arguments modify preference means that you are (denotationally) arriving at different preferences depending on arguments. This means that, from the perspective of a specific given preference (or "true" neutral preference not biased by specific arguments), you fail to obtain optimal rational decision algorithm, and thus to achieve high-preference strategy. But at the same time, "absence of action" is also an action, so not exploring the arguments may as well be a worse choice, since you won't be moving forward towards more clear understanding of your own preference, even if the preference that you are going to understand will be somewhat biased compared to the unknown original one.

Thus, there is a tradeoff:

• Irrational perception of arguments leads to modification of preference, which is bad for original preference, but
• Considering moral arguments leads to a more clear understanding of some preference close to the original one, which allows to make more rational decisions, which is good for the original preference.