Prevents long lying around batteries

The biggest advantage of adapters is the ability for me that almost all batteries are always somewhere in operation.
One can almost use the Power of HR20.
It's just more flexible, whether AA or HR20 / D: flashlights, camping, fishing gear, toys, etc.

Batteries that are continuously operated, end up costing less.
Hold, based on the total life, longer the standard-capacity. You have less frequent reloading.
Unused batteries can be discharged so deeply alone through self-discharge that they are junk.

HR20 / D Cheap batteries are sometimes nothing more than AA assembled with larger shell.
Just the weight can be established there.
Compare times the capacity of good AA with D / HR20, why pay more?
If a dochmal reach its lifetime is disposed only half, rather than a complete HR20 cell.

Skepticism has not been confirmed, keep both the contact springs and the locks. Thumbs up.
Christmas may come.
By frequent use also the service life of the battery extended. I now have any left).