Icebear,
You may have a different style of GMing than me. I like creative uses of spells, and appreciate players taking risks. I don't allow any half-baked idea to become a free add-on feature for all situations -- I make judgment calls with every unusual use attempt.
If a player attempts something reasonably doable with a spell, I may allow them to make a roll. If the idea is new or imaginative, I may even give them a beginner's luck bonus. On any given situation they might succeed, attain pyrrhic success, blow it and lose the spell, or not even have a shot. I don't mind that the designers didn't have the print space or foresight to explain every possible application for a spell. I try not to let that limit me or my players.
I wouldn't imagine for a moment that I'd be boxing myself in -- forced to allow free flanking for the caster and possibly other PCs with every casting of Mirror Image. In rules which I don't deem critical to balance or character play/development, I try not to veto a well-played idea by its worst imaginable scenerio.
And I do try to be consistent. If a player comes up with something new in good faith, I appreciate the effort, and try to incorporate the idea to the extent it's reasonable and makes for good drama. If the idea is something creative but not heroic (and so may get regular usage), the player can expect the results and governing rules to shift around a bit while the new idea gets its legs. I'd openly laugh at a player who tried to call 'foul' on a house rule I was tweaking in good faith for balance.