Smugglers' Forest and the Polluted Swamp were interconnected. Miss Ceris had explained the reason for this place's existence to Murphy before; it was an "alternative route" from Transylvania to the Dark Mountain Range and then through the guarded pass of the Brass Dwarves into the Ligurian Peninsula.
Of course, the name Smugglers' Forest didn't sound very reputable, so respectable folks certainly wouldn't travel through here.
For instance, the official route for Miss Milian, the Crimson Regent, when she went to study on the Ligurian Peninsula, was to first cross Lake Xiuerpres to reach the Kato area, and then take a train from the station connected to the peninsula and travel straight through to Charldo Harbor.
The biggest advantage of this route was its safety.