I read recently that fluorescent clothing was not effective at all at night. If this is the case, how effective is it during the day, and should I stop buying tops that are fluorescent to use on my daily commute?
There are several terms around this subject. A key thing here is reflectivity (or more specifically retroreflectivity), rather than fluorescence. Fluoresence is the property of a material where it absorbs light at one wavelength, e.g. ultraviolet sunlight, but then reflects back light from the visual spectrum. This is most useful in daylight conditions, using the sunlight to create more human visible light.
Everything (well, almost everything) is reflective to some degree, mostly diffuse reflection where light hits a surface and scatters, but this reflection goes in many directions and breaks apart into many wavelengths. Alternatively there is a property called retroreflection which reflects light back to its source, in contrast to mirror (or specular) reflection which just bounces off, as light does with a mirror, in a single opposing direction. Hence a person near the light source, e.g. a driver in a car, sees the retroreflective surface.
Thus in terms of visibility, you don't just need flourescent material at night because there isn't enough light in the usable spectrum, so a yellow jacket isn't going to be any more useful than a white t-shirt. So while they'll both be more visible than something black, in order to stand out in a driver's headlights, you need retroreflective material to actively react, rather than passively reflect.
The question asks how effective fluorescent clothes are during the day, and should he stop buying those tops for his daily commute.
Fluorescent clothing is very effective during the day, but not so much at night. From How Does Hi-Vis Clothing Work?:
And it continues about why reflective material is good for the night, but not during the day.
Fluorescent/Reflective clothing only help you if there is a light pointing at you from about the same direction as the driver is coming from. The little light a fluorescent object will give out its self is not of any use as the car driver has the bright light from his/her headlights reflecting of the road just in front of his/her car.
Given that most people get knocked of bikes from the side, with car drivers pulling out, rather than from the back, it is very likely you will be outside of the car’s headlights beam at the crucial time you most wish to be seen.
So you need good lights!
However Fluorescent/Reflective clothing does help make you more visible and can make you show up a lot better in street lights. It may also help you win the legal case if you knocked of.