Having trouble with this one. What does it mean and what might be some reasons leading to this conclusion. The relevant text is "What We Believe Ourselves To Be"
1. Parfit and the non-reductionist view of personal identity
Parfit rejects the idea that personal identity - continuity and survival over time - involves the continued existence of any irreducible persistent entity such as the Cartesian Ego. On the contrary, personal identity is reducible to continuities - similarities between Derek Parfit at time t1 and t2, t2 and t3 and so forth. Madell elaborates:
IN chapter 11 of his Reasons and Persons (Clarendon Press, 1984) Derek Parfit develops what he regards as a central argument against the Non-Reductionist view of personal identity. This is the view that personal identity is strict, and not analysable in terms of physical and/or psychological continuity. On this view personal identity involves, in Parfit's words, a 'further fact' in addition to any such continuity, even a 'separately existing entity', such as a Cartesian ego. Parfit's Reductionist view, by contrast, is just that personal identity through time involves nothing more than such continuities, and of these psychological continuity and connectedness is much the more important. On this view, what we call (however loosely) 'personal identity' is essentially a matter of degree. (Geoffrey Madell, 'Derek Parfit and Greta Garbo', Analysis, Vol. 45, No. 2 (Mar., 1985), pp. 105-109: 105.)
To anticipate : a replica of oneself has a similarity to oneself and so could be a stage in one's future identity. But there is a time condition on which my relation to a Replica means that my death 'though better than ordinary death, is not nearly as good as ordinary survival'. See 2, 'The branch line case' below. So it cannot be straightforwardly said, without qualification, that for Parfit 'I ought to regard having a Replica as being about as good as ordinary survival' even if at one point he does unguardedly make this claim.
2. The branch line case
The key to Parfit's meaning is to be found in what you will already know as 'the Branch Line case':
In the Branch Line case, Parfit imagines a "scanner" that, at the press of a green button, destroys and analyzes his entire body, including his brain. The scanner is linked to a "replicator" that assembles a molecule-by-molecule copy of him on Mars. He then imagines that the scanner is upgraded to a model that leaves his original body intact, so that there are duplicate versions of him, one on each planet. Finally, he imagines that the upgraded scanner has damaged his heart and that he will consequently die within a few days. Having received this dire prognosis, he speaks with his replica on Mars by interplanetary videophone:
Since my Replica knows that I am about to die, he tries to console me with the same thoughts with which I tried to console a dying friend. It is sad to learn, on the receiving end, how unconsoling these thoughts are. My Replica then assures me that he will take up my life where I leave off. He loves my wife, and together they will care for my children. And he will finish the book that I am writing. Besides having all of my drafts, he has all of my intentions. I must admit that he can finish my book as well as I could ...
If we believe that my Replica is not me, it is natural to assume that my prospect, on the Branch Line, is almost as bad as ordinary death. I shall deny this assumption. As I shall argue later, being destroyed and Replicated is about as good as ordinary survival.
Parfit later explains his view of the case as follows:
It may be slightly inconvenient that my Replica will be psychologically continuous, not with me as I am now, but with me as I was this morning when I pressed the green button. But these relations are substantially the same. It makes little difference that my life briefly overlaps with that of my Replica.
If the overlap was large, this would make a difference. Suppose that I am an old man, who is about to die. I shall be outlived by someone who was once a Replica of me. When this person started to exist forty years ago, he was psychologically continuous with me as I was then. He has since lived his own life for forty years. I agree that my relation to this Replica, though better than ordinary death, is not nearly as good as ordinary survival. But this relation would be about as good if my Replica would be psychologically continuous with me as I was ten days or ten minutes ago.
Parfit does not explain why the survival of a forty-year-old replica would be less desirable than that of a replica produced within the past ten minutes. He seems to imply that the survival of the forty-year-old replica would be less desirable because he has "lived his own life for forty years" and would be less likely to carry on the life that will be cut short at Parfit's death. At the replica's creation forty years ago, he might have finished the book that Parfit was writing then, but he now lacks the beliefs, desires, and intentions that would enable him to finish the book that Parfit is writing now and will not survive to finish. Parfit's judgment in this case thus illustrates his view that what matters in survival is the continuation of that in oneself or one's life which one finds important.
3.The time condition
Parfit concludes his discussion of the Branch Line case with the admission that his judgment is counterintuitive:
... I admit that this is one of the cases where my view is hardest to believe. Before I press the green button, I can more easily believe that my relation to my Replica contains what fundamentally matters in ordinary survival. I can look forward down the Main Line where there are forty years of life ahead. After I have pressed the green button, and have talked to my Replica, I cannot in the same way look forward down the Main Line. My concern for the future needs to be redirected. I must try to direct this concern backwards up the Branch Line beyond the point of division, and then forward down the Main Line. This psychological manoeuvre would be difficult. But this is not surprising. And, since it is not surprising, this difficulty does not provide a sufficient argument against what I have claimed about this case.
Reading this passage, one wonders why a difficulty should have to be surprising in order to be philosophically significant. One rather suspects that Parfit's talk of "redirecting" his concern up one "line" and down the other - talk of a kind that appears nowhere else in Parfit's discussion - reveals an important feature of self-concern, a feature that will help to explain the rationality both of wanting to have future selves and of caring about their fate.
(J. David Velleman, 'The Identity Problem', Philosophy & Public Affairs, Vol. 36, No. 3 (Summer, 2008), pp. 221-244: 225-7.)