When you skewer food the point is exposed to raw meat, and if you marinate on firewire then the entire skewer becomes contaminated with the raw food including any bacteria.
If you leave the point hanging outside of the grill so it will stay cool then any bacteria from the food or the marinate will remain on the skewer and will be reintroduced to the food when you remove the skewer.
Get more detail about Fire Wire Stainless Steel Flexible Grilling Skewer, Set of 2.