Difference between dedicated tag line and 6/7 mm accessory cord

Any reason to buy a dedicated tag line (>$$) as oppose to just purchasing ~65 m of 6/7 mm accessory cord from the local gear shop....other than the store employee might hate me?

edit to add: would be looking at similar diameters for packaged tag lines and accessory cord, I understand the correlations between weight/diameter/tangles

Nope! But you should also consider the Beal Escaper.

both are low-stretch ("static").  main differences are weight and ease of handling. thinner cord = lighter; fatter = heavier.  fatter cord will generally handle better, especially for hauling.  consider how you'll most likely use it, and decide whether weight or handling is the more critical factor for you.  if you go thin and change your mind, you'll have a lifetime supply of cordelette...    pre-cut, packaged, labeled/marketed specifically as "tag line" likely means your paying more for it.  I've seen cheap hardware-store poly rope used as tag-lines, and as fixed-lines on big expedition routes.

Imagine 1/2" webbing

Buy whatever you are willing to rap on over rough terrain.