Such a view seems quite bizarre. Of course, there might be practical grounds for preferring direct effects insofar as a shorter causal chain is more of a "sure thing", less likely to get derailed, etc. If you don't trust the doctor to actually save more lives in future, then you need to factor in the uncertainty of the later possible benefits. But that isn't Kamm's concern here. Rather, it's simply better in principle (for reasons of "fairness") to prioritize the people you can save directly rather than indirectly.
Why think this? Kamm seems to think that to do otherwise is to treat the non-doctors amongst us "merely as a means": (p.148)
I have argued that to favor the person who can produce [extra utility ...] is to treat people “merely as means” since it decides against the person who cannot produce the extra utility on the grounds that he is not a means. It does not give people equal status as “ends in themselves” and, therefore, treats them unfairly.
But this is clearly confused. The utilitarian does not favour the doctor because he has greater status as an "end in himself" -- all people are regarded equally as ends. The difference is rather that, besides his value as an end in himself (equal to the janitor), the doctor also has -- ex hypothesi -- greater value as a means to helping other people, who are themselves "ends in themselves", and whose interests merit equal consideration.
Is there any better reason to take Kamm's distinction here seriously? (For the curious: the earlier "argument" she references can be found on p.112: "[I]t may be said that we are treating A only as a means since we decide who gets aid by seeing what function each person can perform... It is true that here we do not ignore A's needs entirely... Yet denying him the drug because he is not useful, when it is not in his interest to be denied the drug, is, I believe, adequate grounds for saying we are treating him solely as a means." Colour me unconvinced.)