Is it worth wearing a headlamp when cycling as well as having a bar mounted light? Most bike lights seem to be the bar / seatpost mounted variety. Is it worth also having a headlamp for riding?

I go cycle touring, ride in the city and do mountain bike xc trail riding.

I've got a budget enough to buy something decent.
This is always going to be a matter of opinion but here are some reasons why I think the answer is "yes":

Cornering: you don't have to be cornering hard for a bar-mounted light to point the wrong way, and cornering is a time when you really don't want to hit a pothole. 

Signaling to others: if a bus starts indicating out while you're alongside, lifting your head to flash their mirror is quite effective. 

If you come off in the dark: you don't want someone to swerve round the bike towards your head. (my head torch has a rear light as well, thanks to my modifications). 

I also like the fact that when riding with my bar light quite dipped (e.g. on a busy, two-way but unlit bike path), I have hands-free control of how far in front the road is illuminated, and can avoid dazzling people coming the other way while still illuminating the road ahead.  I use a head torch with a narrow beam.