if it isnt thick enough to remain completely rigid, then its not suitable for the job. either your fastners will start to pull through the backing material and cause loose fastners, or complete failure, or it will distort and compromise the surface you are trying to protect. the theory behind the use of a backing plate is to take the applied force and spread it EVENLY over a much larger area.... if the backing plate distorts, it will not maintain an even distribution but cause extreme loading in the area that is most susceptible to failure (around the fastener hole) and none towards the outer edge...... although any type of material may help for a little while, but doing it right the first time will prevent any problems.
some boat manufactures have used high density plywood as backing plate material to save costs, which when new and good seemed ok, but over the years it has failed and needs to be replaced. it has, in some instances, caused extreme damage because of the sudden failure when needed.
so you can make it look good with about anything, but when performance counts you want the good stuff.
personally I use 1/8 stainless or 3/16 aluminum plate, but then i realize not everyone has the tools to work with stainless..... aluminum plate can be worked with wood working tools so not much excuse not to use it.