But whatever change personalized search makes relative to generic search, the opposite change will occur going from personalized to generic. To penalize personalized search when the change is of equal magnitude going either direction doesn't seem fair. As long as the personalizations are just a transformation and not a subtraction, the two options are just mirror images of each other. They have the exact same kind of failure condition. The question is just which one's failure conditions are more likely to occur.
As a counter-example: Suppose your searches are primarily academic. Let's say that whenever you search the term "momentum" you are looking for something scientific- ballistics, elementary particles, whatever. But most people aren't looking for scientific info, so you constantly have to dig and dig to find anything relevant on Google. The point is, if the search technology is too impersonal, you may not find the info most relevant to your interests.
As long as the personalizations are just a transformation and not a subtraction
This is the crucial point. As long as it's accessible, I can get what I want by search refinement either way (but I would imagine that it would be easier for me to refine in the space in which I am familiar than to refine in the unfamiliar space). Giving me the (easy) option to turn it off is a good idea for search.
As a counter-example: Suppose your searches are primarily academic. Let's say that whenever you search the term "momentum" you are looking for something scientific- ballistics, elementary particles, whatever. But most people aren't looking for scientific info, so you constantly have to dig and dig to find anything relevant on Google. The point is, if the search technology is too impersonal, you may not find the info most relevant to your interests.