IMO light and light cycles have just as much of an effect on amphibian species as they do on people. You can observe this by suddenly changing(shortening/lengthening, altering intensity) light cycles, most species will display unusual behaviour.
If someone suddenly turned the sun off you'd be a bit confused! Exposure to light in the majority of animal species regulates behaviour, breeding, feeding, resting etc to some degree, even if they do not require particular kinds of lighting for a specific biological process(for example the requirement in some amphibian and reptile species for UVA/UVB lighting to metabolise particular minerals from food).
In any case I believe a keeper should attempt to replicate an animals natural environment as closely as possible. Many species will be exposed to light of some sort for at least part of the day, so that should be replicated as well.
Daylight fluro's arent too expensive, dont use much power and dont give off too much heat. Over here small(8-12 watt) daylight CFL's only cost 1-2 pounds, dont know what prices are like over the pond(probably cheaper!) but it wouldnt set you back much.