as I'm sure you're aware, PD is able to defragment these "offline" files, whereas JkDefrag cannot.
No, never heard of "offline metadata". Can you tell me more? Where is it stored, and how have you determined that JkDefrag cannot defragment it? What does the JkDefrag.log file say, which files cannot be defragmented?
JkDefrag can defragment and optimize 99.9% of your disk, there are a few system files that JkDefrag cannot defragment/optimize (and a few more on Vista). The decision you have to make is how much money you want to spend to defragment the last 0.1% of your disks. You won't notice any system performance increase, I can promise you that.