Hi Black_nail,
Having poly on the back and sides provides a smooth surface for tape and glue to bond to. And with more surface area to bond to (compared to just lace) you get a stronger bond. The poly is not detectable to sight. With your density, 60-65% it will not be visible. But the trade-off (there's always trade-offs in these choices) is that the poly can be detectable to touch. So if you're concerned about a woman running her fingers through your hair you might want to go with all lace. Also, people that wear all lace say that it's not necessary to have a bond that lasts beyond 2 weeks. They like to remove their units after 1-2 weeks, clean their scalp, and do a fresh attachment. They say that the lengthy bond time causes their scalp to itch and they're not comfortable with having a unit on for more than 2 weeks.
Regards,
John
http://:toplace.com/