The key difference is that the fangs aren't a physical manifestation of magic the dagger is. Theirs not much difference between creating a dagger out of magic and stabbing someone and creating a ball of fire and killing someone, the daggers just slower.
on the other side where's the difference to taking 15 minutes to walk down to a shop and buy a dagger and making a 15 minute conjuration of a dagger? it is a difficult question...
but I have another question along that line - how about summonings? I mean - a wizard tries to summon a demon - he gets the containment spell alright and reckons, he will need it until noon tomorrow. let's say our wizard is not so good at binding after he summoned the nasty. let's say he fails at binding, but that is ok, since the containment spell still has some time good. then he is distracted, whatever, got called to a friend in need - pick your scenario - and all the time the clock is ticking.
meanwhile our demon is somewhat imprisoned in the wizard's conjuring circle, but get's his laugh when the little wizard tries to bind him. now the question: can a wizard, who has not bound the demon/spirit/whatever make it go away? say the demon looks at its watch and thinks it has nothing important to do than to wait for the containment spell winking out, and eating the wizard then, or whatevs demons do.
our wizard comes back totally exhausted from the emergency, 5 minutes to noon, just remembering there might be a demon who is set loose really soon.
feasible scenario?