adam has a good point. If you know it was wet, do you know if it is rotted.
the reason for the 2 cuts is because it typically looks better to have symmetry. If you consider that, you would end up with 2 3" cuts, one being on either side. Small cuts show differences in the width more than a larger piece.
Ultimately, it is a matter of aesthetics though and since this is your house, you can ignore the guy that has been doing this for a lot of years and has had people tell him what looked good and what didn't or you can make your own decision on how you want it installed.