It's funny you should ask this, because I've recently wondered about the same issue. My daughter is also ten, and still insists on sleeping with a light on (brighter than a nitelight, but dimmer than "regular" lighting -- we use a 20-40 watt bulb in a novelty lamp that emits a soft green and yellow glow).
After talking this over with some girlfriends (including one who is a pediatrician and one who is a child psychologist) I decided to let it be. My daughter is obviously afraid of the dark -- and depsite the fact that her fear isn't "rational", I've come to believe that I should respect her wishes.
I HAVE noticed, however, that some of her friends tease her about this during sleepovers. Part of me was hoping that this would convince her to try sleeping without the lights...but part of me is proud of her for not giving in to peer pressure -- and not even feeling embarrassed about her need to sleep with the lights on.
Assuming that they outgrow this phase before college, I can't really think of any reasons to be concerned...unless you are concerned about "wasting" electricity as some other commenters suggested. On the other hand, if your child needs this, I wouldn't think of it as a "waste", but rather as a temporary necessity.