My boyfriend's full name is
Thomas, but he prefers to go by
Tom. His family has always called him
Tommy, though, and so does anyone he went to high school with...in spite of the fact that he tried to get people to start calling him "
Tom" when he started high school. For him, "
Tommy" was a nickname that he definitely outgrew, and I think it's the sort of name that it's percieved one
should outgrow. I can't think of any adults (that I know personally, at least) who'd have no problem with going by "
Tommy", and with that in mind, I don't think that it's substantial enough for a full name, nor is it professional enough. A
Tommy would be thought of as perpetually five years old, and he'd likely feel like it, too (I know my boyfriend feels completely childish and embarrassed when he's called "
Tommy").