It is called downthrust, not down incidence. Downthrust is used to prevent excessive climb under power that may cause the airplane to stall. In models, it is very common in trainers, which are normally set up to be rather nose heavy, in order to give them good pitch stability. Such a setup needs a bit of downthrust to avoid having to use "down" elevator input to prevent climbing too steeply and stalling when a lot of power is applied, when the airplane is trimmed for level flight at slow cruise.

A couple of degrees of downthrust will not cause the airplane to go down, unless the CG is far further aft than normal, in which case it will require little or no downthrust. If the airplane noses down under full power, it needs less downthrust... (much less).