On another forum it was stated a number of times by Bloxham, Feller and Kamradeniz and I have seen it mentioned on a few individual hair loss surgeons' websites.
Basically the idea is that the scar tissue left behind distorts and scars the remaining follicles, making future harvests more difficult. I haven't seen a hair loss surgeon (FUE specific or otherwise) refute this claim.
A counterpoint is that the surgeon may be able to take more density with FUE than with strip because he's not concealing a scar, though obviously this is less of an issue with a good strip line.