What are the advantages, if any, for a Tennessee handle spinning rod?

No threads from a reel seat, very comfortable but the most impotant advantage is balance. Not every spinning reel weighs the same so a Tennessee handle allows you to mount the reel where ever you want it on the cork, a heavier reel you may want it more toward the front, a lighter reel you may want further back and vice versa depending on how you want the set up to feel.

The reel can be placed at the exact spot to get the balance where you like it, without using any other weight or balance systems.

Its lighter. The cork or graphite is the real seat. No metal ring. Nothing.

Wish all my rods were Tn handle rods. I have way better feel also, as the reel is touching the blank.

Pete

There was a time when the T handle was popular because reel seats were metal. That meant heavy and cold. Tape on the cork was much more comfortable. New grip and reel seat designs over came that problem and so went the T handle. I still have one, a 7' drop shot rod for real light line. It's a great rod.