The issue is how they are used. If they are used as motivators or as behavior management tool then the child does not learn self motivation or self respect.
I ran a session with some year 2 kids the other week that highlights this issue. A girl called 'Bethan' was literally in my face the whole session. She wasn't engaged in the learning, was showing off, was not working as part of a team or respecting the others around her. Yes, every answer she gave was 'right' (and from the outside probably looked great) but she was NOT engaged in the work. At the end all the children left proud of what we had achieved (saving animals from a flood is no easy task) but Bethan walked straight up to me and demanded a sticker because she had done 'good work'. She was mortified when I explained that I didn't give out stickers so she sulked and actually renegotiated with the TA her 'reward'. She wasn't satisfied with a pleasure and happiness from self. What a dangerous thing for a young girl to learn? That reward comes in the form of a material object or an emotional stroke. Aha, that's why she wasn't truly engaged because it was all about the sticker. Will she remember the session? probably not!
Often at the end of a session the teacher will ask me to pick one child that has done 'really good work'. What a ridiculous thing to ask. You see the children suddenly sit up. In the past, I unfortunately felt pressured to do so and noted the immediate upset of all the other children who were told, by the message given out, that their work was not 'good'. They don't remember the learning adventure we have been on just that they didn't get picked. Great of your child is the one always getting the stickers, or so it seems. I stopped this in its tracks and went away and read up about it.
It is harder to motivate children without a reward system but hey, its harder so I best get on it and work that little bit harder to really engage them in what I am doing. I have seen teachers wave stickers in front of classes saying "now who is going to behave and get a sticker'. well let me work hard to make my sessions that have pace, are engaging, purposeful and relevant so that reward, passion, motivation is created so that the children become curious, independent learners rather than passive or, worse, only participating for reward like poor little Bethan. She missed out. Whilst the rest of us were busy exploring the rainforest she was busy trying to second guess what I would think was 'good work'.
Don't get me wrong stickers are fun, but in the classroom? sorry but they are lazy, short term answers.
Read Alfie Kohn/Ian Gilbert.