Agreed. No need to keep the old glass. No one seems to be able to glue it back in and make it stick.
The new top comes with the glass and defroster.
A note from my experience: Make sure your top mechanism is working before you tear into the job. Mine isn't and I don't know if I did something, or if it hadn't been working previous to replacing the top. (I did have to dry out the computer in the trunk fender well. With the window out in the old top it managed to form a lake of water around the computer and that can cause many failures.)
Best of luck!
Bill