A longer-than-average wheelbase is good for fully loaded touring. It gives room to adjust the fore/aft position of your rear panniers so that they are centred over the rear axle and your heels don’t clip them. A longer wheelbase also means less likelihood of toe overlap with the front wheel.
Chris is a smaller fellow with large feet, who carries the lion’s share of our gear. He had problems on his Devinci with heel-clipping when loaded, and the toe-overlap was extreme. This left no options for adjusting the cleat position on his shoes, and poor positioning caused calf muscle problems.
Surly sensibly uses 26” wheels on its smaller frame sizes, and these sturdier wheels are now an option on larger sizes. Chris’s 54 cm frame with 26″ wheels provides generous clearances.
Many “touring” bikes, such as the Devinci Caribou, are really “sport touring” bikes, meaning their wheelbase isn’t any longer than any other road or hybrid bike’s. Devinci, like many manufacturers, uses 700c wheels on all their Caribou frame sizes, making the smaller frame sizes crowded. Devinci puts lugs for a third water bottle mount below its down tube, but even a small water bottle doesn’t fit there without rubbing on the front tire. The Devinci Caribou has serious limitations for extended travel.
The longer wheelbase and other frame geometry factors contribute to the bike’s wanting to keep going straight, so it’s relaxing to ride on a long tour. Some complain a true touring bike is sluggish to ride around town and on day trips. I find riding the unloaded Surly in town just makes me feel relaxed.