As
gios mentioned above, think about this when you're driving (if you do). What works and what doesn't.
Reflectors are very beneficial in marginal conditions like dawn/dusk or when it's overcast or raining.
Many developed countries now require reflective sidewalls on tyres. These not only tell drivers 'heh, there's something here' but what it is and what direction it's going. Very beneficial and much better and less annoying than plastic things in spokes.