I agree with God, though with less hostility. THC is fat-soluble, as we all know. You "might" (the might means speculation) lose THC in the bong, and this is, of course, what we definitely don't want to happen, since drinking the bong water is just downright disgusting. This means that you would just be losing THC.

With that said, I have heard of people successfully using other liquids such as wine, etc. in their bongs, and there are products with which to flavor your bong water. I would not suggest using oils instead of water, but if you feel like it, and you're okay with the bad taste and the potential to lose THC, be my guest. After all, you payed for it, right?

Damnit Grim, you beat me to it.