The Batonlite isn't 42 lumens. There is no direct translation between lumens and CD, MCD, or lux. It's actually somewhere in the vicinity of 7.4 lumens, extrapolating from the Overall Output test results on flashlightreviews.com. Roughly the same as an ARC AAA-P, in fact. It's a good "keychain" light, if large, with exceptionally good runtime. Could be considered an alternative to the minimag as well for many individuals, and has similar carry options. LEOs often value it as a ticket-writer light. For tactical purposes, looking at it purely from a lighting perspective, it's only useful for close-range threat assessment. Any other defensive value it has is as a kubotan.
The use of a light like the Streamlight Batonlite or the Streamlight Stylus as a kubotan can be effective, but it requires training. In any case, as with any other light, use of a flashlight as an impact weapon is something to be considered very carefully and trained for (among the considerations: do I really want to risk screwing up this expensive light when a much cheaper and less delicate instrument could do the job better? Also, if it gets damaged in a fight, at what point does it cease being useful?) I'd consider the Batonlite a rare exception to the rule in that it's designed with self-defense in mind and is damn near indestructible (moreso than most other tactical lights) but generally, flashlights are usable as impact weapons only as a last resort.
Another consideration is that you can use a flashlight *in conjunction with* an impact weapon or kubotan that will do a better job. Well-timed direct flashes to the face with something very bright in very dark conditions could help throw an attacker's aim or timing off, for example. You can easily dazzle yourself at the same time, but at least your finger is the one on the button (so you'll know when to blink.) You also get the option to dual-wield if you're going the impact device route (not an option most people would want to consider in most cases, but there are uses for such a tactic.)
If you really want to use any sort of pain-compliance device or impact weapon for self-defense, though, you're probably better off getting something lent better to the task. There are some really good kubota out there that will work FAR better than the Batonlight, and almost any real club or baton will fulfill the role better than a maglite for a number of reasons. There are even times when other common non-weapon objects can be used as weapons more effectively; for example, pens can make extremely effective kubotans and many can also be used as stabbing weapons if you're in a severe SHTF scenario. Anything with some mass (like a coin purse) can be adapted for use as a sap, or have cord attached in some way to be used as a mace. Strong cord can even be used as a garrote, and can be carried in simple and sensible ways (see the Chain Knot, or simply get a sturdy retractable lanyard and put keys on it.) DR's site has some interesting ideas, and there's a wealth of information on the subject out there.