The way we see it, stirring pasta is a good idea for two reasons. First of all, dried pasta has a tendency to sink to the bottom of the pan right after you add it. The hot metal bottom is significantly hotter than the boiling water, and the sinking pasta can stick and scorch when it hits.
Secondly, unstirred pasta has a tendency to clump together. Pasta releases a lot of starch as it cooks, which normally is diluted in the pasta water. But if the noodles are very close together, the half-dissolved starch becomes a glue that binds neighboring noodles together. Good luck getting those noodles separated!
We usually give the pasta a stir right after adding it to the pot and then again a minute later. Once the water comes back to a boil, there's generally enough motion to keep the noodles separate and cooking evenly.
Anyone have a good horror story of forgetting to stir pasta?!