For the same reason I wouldn't recommend somebody named
David going by
Davey as an adult, or a
Nicholas going by
Nicky. Some nn's are just better on children than adults.
This is subject to regional tastes, though. In the South, it's quite common to see men called
Tommy,
Billy,
Bobby and
Ricky etc, but up north you don't see it as much and it seems not to be the thing to do.
I know many Thomases (including an uncle) but I know of none who go by the full name in their daily lives. I do know a couple who have, say, business cards that identify them as
Thomas professionally, but even they are not called
Thomas socially.
I know a couple Sams, but none of them goes by
Samuel. (Unless their mothers are mad at them! lol
My husband is named
Phillip, and his parents never called him
Phil and never wanted anybody else to. But most everybody he meets, even though he would introduce himself as
Phillip, takes it upon themselves to shorten it to
Phil, and he has jsut about given up trying to correct them. So, with family and a few close friends he's still
Phillip, but most of his work buddies and such call him
Phil. They kow what his preferred name is, (if I call his work and ask to talk to
Phillip, they know who I mean) but it's like they can't stop themselves from trimming it down.