It's a catalyst. It provides energy to make the reaction possible. Each and every single reaction that creates D3 IN THIS MANNER requires UVB. When the UVB stops reaching the turtle, this reaction stops. It happens in humans too.
http://en.wikipedia.org/wiki/Vitamin_D
Literally, if the sun shines on your skin while you are walking from your front door to your car, Vitamin D synthesis takes place. When the UV light stops hitting your skin, the reactions stop.
D3 can be derived from other sources, but there are several other known benefits of UV lighting. If it's so easy to provide, why not? That's my reasoning.
Also, adult turtles are much more forgiving than growing hatchlings. The problems with trying to raise hatchlings indoors without UV lighting are too numerous to mention. Can it be done?..sure. Should it be done?..no.
While working at a vet, and even just in herp circles I've heard some horrible stories of inadequate husbandry. Even reading older herp husbandry books make you cringe. One that comes to mind is something to the effect of, "...keep your NA Wood turtle in a small, shallow dish of water and give it little bits of whatever crosses your table." Hmmm, cheeseburgers, ice cream, pizza, potato chips...there are no limits.
It has only been a fairly recent development that people have begun breeding turtles in large numbers and raising hatchlings to adult. Many inadequate husbandry practices have been revealed due to this. I still think that, in general and out of all species considered, chelonians are the most difficult herps to keep happy and healthy. Raising hatchling box turtles in particular is very difficult, in my humble opinion. Are there stories of simplicity?...sure. However, we should strive to do the most and best, not the least.
It's all or nothing. If you're keeping a box turtle (or any pet), it is your job to provide EVERYTHING that they need! I know that you're not recommending withholding what they need, you're trying to determine if they actually need UVB/UVA. Well, they may not always need it to survive but I think it's clear that they need it for optimum health.