Ok, produce flame, as well as a few other druid spells of note, call lightning and storm of vengeance, are essentially buff spells that grant you an attack for the duration. Specifically in the case of Produce flame, since the range is 0 ft., and not Touch, it cannot be considered "holding the charge" as on a touch range spell.
This is why spells like Produce flame and Call Lightning are generally better used as pre-combat buffs. ESPECIALLY Call Lightning, with it's 1 round casting time, and minute per level duration. After the initial buff, you can take the specified action to call down a lightning bolt, or throw a gob of fire on the baddie.
For a REALLY fun effect, one could actually cast Produce Flace, then an Inflict spell, both ahead of time. Hold the charge on the Inflict until combat starts, then hit baddie with both at once! They stack and everything!
Anywho, that's my reasoning behind it, but I'm entirely happy to go with whatever you feel suits you game.
