Once you start offering rewards for things like participation, you will find that some students refuse to do anything - homework, speaking, writing, opening their books, etc - unless there's a reward. Which is why I don't offer rewards for participation. Moreover, I think that rewards of this kind may actually do more damage than good, e.g., shy/quiet students who never won a prize might get demotivated. Or there might be a race to shout out an answer simply to be seen to be participating, which in turn might lead to quantity rather than quality of input. Learning is not a race and should not be treated as such. I prefer to convey the message that the ability to communicate is reward in itself, although admittedly many students would prefer an actual prize!
I do, however, use rewards (stickers usually) with my pre-schoolers. They get these for good behaviour rather than participation. In the four or so months I've been teaching these particular groups, behaviour has improved a lot. I don't know though whether this is because the children now know what's expected of them or because they think they might get some stickers. In any case, the end result is the same - their behaviour has improved - thus I am happy to keep incentivising them in this way.