When I first saw the trilogy in chronological order I felt the DeNiro scenes were somewhat dull - something I never felt when they were interspersed with the Nevada plotlines.

After repeated viewings I grew to appreciate the older scenes much more and now feel they are equal or superior to the rest of GFII.

I think that calling them "gimmicky" is a big stretch. It makes perfect sense to establish the beginnings of the long-standing relationships that Vito develops.