Good question! Let's get this started!
Firstly, I'm a "battery vampire."
http://www.candlepowerforums.com/vb/showthread.php?405759-Battery-Vampires-2015
When you become a battery vampire, you simply MUST know this stuff or you risk yourself and your electronics.
Ok.
1. Why is low voltage variation good in a driver?
Simple. For there to be "overdischarge protection" in the flashlight electronics, the driver MUST cutoff before a certain value is reached. This value isn't really cut-and-dry. Some protections cutoff at 2.9v, 2.8v, 2.7v, but by 2.6v the protection really should have kicked in by then in a single battery lithium ion light or device. 2.5v is an overdischarge, you damage the battery and increase the chance of problems. Charging such a battery (below 2.5) is risky and should be done OUTSIDE, imho. So say we cut off a single battery at 2.9v to be on the safe side. A 2 battery in series light then should cut off at 5.8v.
For this to work, we CANNOT use battery extenders or else the light will now overdischarge the two or three batteries if our cutoff is only 2.9v. You see?
2. Why is wide voltage acceptance good?
Wide voltage acceptance allows you to use multiple chemistries (but not at the same time, you could run alkaline OR lion for example,) varying numbers of batteries, or even hack the light or device to another battery type that it wasn't really meant to use. You must know what you're doing if you play these games. Hook it up backwards even once, even for just a second, and you've now trashed your light, possibly your battery(ies) and maybe hurt yourself or lit a fire.
My favorite wide driver is 3v-18v. It is safe with one lithium ion as it will cut off at 3v, which is a little high but definitely safe to charge after that. It gives me the freedom to run 4 series if I want, but now I am the guardian of my safety. If I overdischarge, I can check with a multimeter before charging so I can no whether or not to proceed.. I can hook to power tool batteries and active automotive electrical systems for power. Though auto elecs are nominally 12v, when the charging system kicks in the electrics must tolerate up to 15ish some odd volts. Having the overhead of 18v rated max allows you to vampire active vehicle electrics and 12 volt standalone auto batteries no sweat.
3. Why is wide voltage acceptance bad?
You now lost one layer of protection that's there to protect you. You will trash your lithiums if you run four batteries down to 3v combined. You, the end user, must know this stuff. Never recharge lithium ion that you've overdischarged (2.5v and less.) The chance of smoke and fire during charging just became a real possibility.
4. Why is narrow voltage acceptance bad?
You just lost all of your flexibility. You might be stuck with just alkalines and NiMH. You might be stuck with just lithium ion only, and a set number of batteries at that. You might have a light that only works with 3v CR123ax1, but not 4.2v RCR123.
Good luck, Prepped. Stay safe!
Any more thoughts? Questions?