I would imagine that a primitive spider layed eggs, and they were probably covered with some sort of gooey, yucky stuff, with not much purpose except that it just came out of the spider.
Suddenly, a mutation developed and that gooey, yucky stuff, which was some weird organic chemical, now had a free charged end on it, which would cause it to behave like a polymer, and form fibers. These fibers could spread out around the nest.
The beauty of these fibers is that they are sticky. Mama spider suddenly finds that there are trapped bugs, right there in the nest. She doesn't even have to hunt. Yum. If she were an intelligent creature, she would no doubt recognize that God suddenly started sending mana-bugs from heaven.
She didn't intend to cover her eggs with sticky fibers, it just happened that way, but her eggs survived because hunting was easier.
Generations later, some messed up spider didn't know she was supposed to stop secreting fiber when she was done laying eggs. This poor helpless thing was done laying eggs, but kept secreting the stuff that covered them. Poor thing was wasting all that energy because of her defective mutation that caused her to spread extra threads around the nest, but at least God took pity on her, by sending more mana bugs that were now all over the nest, instead of just near her eggs, and so her and her offspring, who were similarly cursed, prospered.
Meanwhile, the glands that secreted the fiber got a bit elongated in one spider. This poor creature didn't have any hope of covering her nest with the sticky fiber. The elongated glands made it come out in long threads instead of a fibrous mass. She just ran all over the nesting grounds laying out these threads. Fortunately, the stuff was strong, and enough bugs still hit the area that had thread in it, so even though her glands were all swollen, and she could only produce threads instead of a sticky mass, it was still good enough. In fact, it seemed to use up a bit less energy. Her offspring flourished.
And so it goes. Each step was a tiny change, that gave the spider just a bit more advantage than the last one, or at least didn't cause the species to go extinct, untill another mutation managed to make it all better.
Is that really what happened? I have no idea. I do know that a fully formed spinnerett would not be necessary to start making use out of thread spinning. A partially formed spinnerett would still do something useful for the spider.
My objection to ID as a "scientific" theory is that it is, as someone else has already said, an argument from ignorance. I can't see why this is useful, therefore it is not useful. If the ID people could somehow prove that transitional forms were non-existent or useless, then they would have something, but they can't do that. All they can say is either, "we haven't found transitional forms" or "we're pretty sure that the transitional form is useless" At best, it's a hypothesis looking for experimental evidence, not a theory at all.