Its not a true neutral t-case the front and rear output shafts have to turn at the same speed. So if you take the rear shaft out no driveshafts spin.
When you have the T-case in Neutral, with the rear driveshaft connected, both driveshafts spin at the same speed, this causes just enought friction to spin the intermediate driveshaft, which isn't a good thing for some bearings. putting it in Gear stops the intermediate shaft from spinning. The only thing i haven't figured out is why Suzuki recommends 2nd?