Light_brown_hair is for hair in an ambiguous zone that is brown but too light to be under the same tag as brown_hair but not blonde enough to really be blonde_hair, either. There's no point mixing the tags together since light_brown_hair exists because it's not what someone is likely to be looking for when searching for brown_hair. The visual difference should speak for itself, really.
Also, there shouldn't be a need for an umbrella tag just because they both have brown in their names. It's just as related to blonde as it is brown. That's the nature of colors.
We have light_brown_hair for the same reason we have aqua along with blue and green. These two girls (post #996631) (post #1074983) don't have the same hair color, and it's a lot more than a slight variation. The tags should reflect that.
I would support retention of the light brown hair tag, I do feel there is a middle ground between what one would label blonde and brown, and this tag is intended to cover it. It's also not like this tag is new either, it has been around for over 3 years so it isn't like this is a new thing either.
As for tagging light brown both blonde_hair and brown_hair, I feel that is actually a bad idea, as most would expect brown_hair to be a darker color and using blonde on it is just creating a lot of unnecessary noise for the brown_hair or blonde_hair tags. As for overlap, the light brown works well because it provides a barrier between blonde and brown (so one does not tag images both blonde and brown for the same hair color, but goes through blonde and light brown or light brown and brown), as well as helping to differentiate light browns from dark brown hair colors or in the case of blondes helping to differentiate darker light browns from very light browns and yellowish hair coloring.
Colors that are ambiguous have been tagged with all possible tags since forever. With ambiguous colors, which can be quite subjective if you have no color picker at hand, it's always better to tag more than needed than to omit a tag people may be searching for. On the other hand, even many contributors cannot distinguish between brown hair and black hair. I think we're talking about different sorts of light brown hair here, so here some examples, most of them currently tagged light brown hair and what I would normally tag them:
But I'm not as in favor of dark brown hair because we already have enough ambiguity in trying to determine black or brown for some characters. I don't think dividing that particular line into two separate lines to fuss over would help.