Does a preteen "need" a cellular plan? Probably not. Does anyone "need" a cellular plan? I could argue that we don't since we made it for a very long time without them.
Anyway, all I can do is give you my situation. My 11 year old has an old iPhone 4 right now that makes calls and does messaging fine. Anything else is a struggle due to the age of the phone. I am getting her a 5s and she will be thrilled until she can get another handed down phone from mom or dad. I like for her to have cellular ability because she is involved in after school activity and such.
Everyone has a different situation so you will have to decide if they "need" a cellular plan. I "want" my preteen to have a cellular plan, but she doesn't "need" to have it.