There is absolutely no reason not to use it. Raw pine is susceptible to decay and animals such as horses and mules love to chew on it. The only real problem that would be a concern would be ground contact and that can be solved by raising the wall off of the ground a little and using treated post or light poles for post. Poplar, even though it is considered a hard wood, is more susceptible to decay than pine. Oak, though a stronger wood is not that much better a choice, bugs love it about the same as the rest, but the animals will leave it alone. A good barn stain on the outside will take care of the weather and bees, and careful planing in regard to the ground will take care of the rest.