This last version is a complex sentence. It lets us say Dickon is doing two things at once:
- Dickon is coming in
- And he's smiling his widest smile
And it also suggests a hierarchy: the most important event is “Dickon coming in”. His smile is not the focus—it’s a bit of extra detail.
(Also, the power of clauses means the supporting event can inherit the subject ('Dickon') from the main event, which makes the sentence efficient.)