I think a micro USB rechargeable flashlight for a car makes a lot of sense. It's just far easier to charge up a device by plugging it in to a common charger that you already need for your smart phone.
As for leaving it in a car, I think that's fine for most of the year. Lithium-ion 18650 cells list a safe storage temperature from -20C to +60C.
On a hot summer day in full sun, it's possible (perhaps even likely depending where you live) that the car's glovebox will heat up to more than 60C. That won't cause your battery to explode or anything, but it will likely cause it to age quicker, and lose capacity. Not a big deal if you replace the battery every couple of years, but it might not last 5 years.
If you live somewhere that regularly gets below -20C in the winter, that will have an impact on how long you can run your light on a full charge. You'll get about half the run-time at -20C than you would at normal room temperature. If it gets much colder than that, the light might not even turn on. And, as with heat, it's possible that extreme cold might cause the battery to age prematurely. Oh, and don't charge the battery if it's below freezing! That is unsafe.
For best safety, use a protected 18650 cell.
Okay, for bestest safety, don't use lithium batteries at all, and use something like rechargeable NiMH Eneloops. Same temperature range as lithium-ion, but a very stable chemistry. But, you likely won't find a flashlight with built-in micro USB recharging.
