for DIY builds I prefer removable batteries so I can adapt to driver/LED.
Even if the light isn’t modded or a hotrod, I can adjust the output with the battery I put in for each trip/journey/use case. if I put in a tame Panasonic cell, I know the light will have less output and more longevity. The same hotrod DD driver can be much more powerful if I put a Samsung cell in. A battery can therefore also give me flexibility.
for the “perfect” use case and the “ideal” light, they can be built-in. i.e. keychain lights. bicycle lights I won’t mod, headlamps, etc