This was requested as to make the 'real time' view of 'today' statistics less 'frightening', because by default piwik will show zeros for all hours in the future. WE could instead add a parameter to hide these hours, allowing to show only stats up to now.
The advantage of doing in the API is ease of use for API consumers: they don't have to do the timezone logic themselves.
Instead of an option to show/hide, maybe it could be a time (or maybe date/time) formatter? IIRC there was a request for 12-hour time format.
(In ) Fixes #2316 - Disabled by default in API for Backward compatibility, but enabled in the UI as it slightly improves readability