Yes it's like the difference between kinetic energy and potential energy. A rock at rest at the edge of a cliff has no kinetic energy, but a lot of potential energy.
I guess Pirsig was making a point about value and not about potential value, which is leading to a larger point. In the moment, value is all you have. Potential value is a future value that requires time to realize.
I have two separate reactions to your suggestion that it's important to remember potential value, the Zen reaction and the Writer's reaction.
Even though Pirsig wasn't going for Buddhism, it's fairly consistent with Zen thinking to make observations about the momentary values of things. My Zen point is that the purpose of meditation and much of Buddhist practice is to see only value and to eliminate potential. When you learn about how to meditate, you learn how to stop your mind from thinking about the future and concentrate completely on the here and now. Remembering something's potential value is antithetical to this way of thinking. Not because it's untrue, but because it's un-now.
My writer's reaction is that while it will always be important to remember things other than the point the author was making. Is adding your point to the author's words helping us to understand his? If Pirsig reminded us about potential value in this sentence, would it improve this particular narrative? Does it open a path that gets to his larger point either more efficiently or more effectively? It might, that might be part of Pirsig's thesis. Or it might be taking a narrative path that doesn't lead to the same place.
It's important to see not just the value of the quote itself - the truth or completeness of an individual passage - but to also remember the potential value of the words as being the destination of the story -- where they lead and where it ends is as important as where it was at any given moment.