More driver space is better suited for buck and boost engines, and overall a wider variety of driver designs.
Luckily not all hosts require retaining rings. For example, an UltraFire SK98 O:) fits ∅20mm H1-A drivers like a glove.
I think certain flashlight designs could get some improvement. UF-1405/1504 torches could get upgraded copper/aluminium pills with room for ∅22mm driver, there are many boost and buck engines available in that size.