Limax maximus is omnivorous. It is a detrivore, cleaning up dead plants and fungi, and a carnivore known to pursue other slugs at a top speed of 6 inches (15 cm) per minute. Source
So I would say they do play a beneficial role to the soil and soil level environment due to cleaning up dead plants and fungi. I would presume they aren't detrimental to the soil fungi that interact with the plant's roots, just the surface fungi.
Aha! Excellent reply...It's all about a part of the food chain; and natural balance of things. Of course, they would be of purpose!
Many thanks for the informative answer. This earns an upvote! Sadly, I have no SP. I certainly wish there was a solution to this. But, all's well.
Peace.