Let me pose this question: if you give them the choice, and *they* choose the light/warmth, how is that a bad thing? In the wild, toads have the choice to burrow or stay above ground, to move into a drier or wetter area, and to move into the sunlight or into the shade, right? I have seen a picture of toads stacked up on top of each other directly under a spotlight, and sometimes my own toads go and sit on the warm rock under the light of their own accord. It seems that at least sometimes, they prefer it. Clearly, you don't want to go overboard with it, but I don't really see the harm in using a low-wattage spotlight in one part of the tank and seeing how they respond, especially in a room that is not very bright in the first place. It seems to me it would probably help reinforce the natural day/night cycle, if nothing else.
One the topic of the "toads are nocturnal" thing, I have a couple of thoughts about that. It would seem to me that if these were strictly nocturnal creatures, they would be repulsed by/hide from the light rather than being drawn to it. Also, on the day I found my toads, they were hopping all over the place in the middle of the day, which doesn't seem like the behavior of a truly nocturnal animal either. I have seen information that says they can be active during the daytime as well:
Fowler's Toad - North Carolina
Also, this study specifically focused on Bufonids (bufo boreas) and their tendency to thermoregulate in the lab or in nature by basking:
ESA Online Journals - The Role of Behavioral Thermoregulation in the Growth Energetics of the Toad, Bufo Boreas
Some other members here at Frog Forum have also commented that basking seems to promote rapid growth, which I am also seeing in my toadlets right now. Some discussion in this topic on the subject:
http://www.frogforum.net/toads/8993-i-love-toads.html
Anyway, that's why I'm using a light. It seems logically sound to me, and some others seem to have success with it as well. As long as you're not overheating your toads in a place where they can't get away, or using too strong a light, I don't see the issue.