A sleeping-bag liner can make sense even when the bag itself is already comfortable. The TETON XL Sleeping Bag Liner adds a separate layer next to the sleeper, which can be easier to manage than cleaning the whole sleep system after every trip.

The XL designation is worth checking against actual measurements. Extra fabric is only convenient if it fits inside the bag without bunching around your feet or shoulders.

What I'd Compare

XL liner format.
Designed as an accessory for a sleeping bag.
Separate fabric layer rather than a replacement sleeping bag.

Confirm the liner's dimensions and opening arrangement. You want to be able to get in and out without turning a simple bedtime routine into a wrestling match between two sets of fabric.

Cleaning Convenience Versus Another Layer

The contact-layer role is the strongest reason I'd consider a liner. Read its care label and compare the washing routine with your bag's. If the liner is easier to clean, that can be useful for repeated travel or camping.

It also adds another item to pack and another surface that can twist when you move. Try the combination before a trip. A liner that works beautifully in a roomy rectangular bag may feel different in a closely fitted mummy bag.

Don't Invent a Temperature Boost

There is no verified warmth increase in the product description used here for this particular liner. I'd choose it for fit and the contact-layer role rather than assume it makes a marginal sleeping bag suitable for colder nights.

My take: the XL makes sense when a smaller liner is restrictive and your bag has room for the added fabric. If you dislike layered bedding, the convenience may not outweigh the extra handling. Buy it to solve a specific sleep-system problem, with dimensions checked on both pieces.