I don't weild my shirt or sock, I wear them. If they were in my hand, I'd give it to you, but on my body, grease would just make them messy, slimy, and, well, greasy.
Most shirts will stay on when slimy, even if you're turned upside down and shaken. The same with pants, especially with a belt employed.
I agree that the Rogue would get a bonus to EA, but in the case of dropping objects, it must be in hand to be consitered wielded. Otherwise grease is just a freaky-weird spell.
- Kemrain the... Greasy?