If you cast light and continue meleeing, the mob will beat on you, but with a reduced atk rating as mentioned in the other postings. But if you cast light and step back, the mob will take off in a random direction looking for you - generally, it does not find you, if it does you step back again.
Consider fighting someone with a pet. Flash the pet, and it is useless for the duration of the spell - after that it probably will come, but by then it might be alone. You want to consider carefully doing this in a crowded area - the mob you flash might bring back a number of friends.
I have used light as a poor man's crowd control technique in Marus Seru with a tank who understood the technique - flash the add and step back. When it comes back, do it again. This works until the mob comes back with an add. If it never does, you can kill it next.
And at low levels, the few attack points that light debuffs the mob (which is specified as 5 in the lowest light spell, and not specified here) really make a difference in how often the mob hits you over the course of a fight.