Waste of money... argueable. However, the potential implications that this "face" might have had would have been phenomenal. Relatively close to that "face" are also structures that some believe to be pyramids and at admittedly, do somewhat resemble pyramids. Now imagine what this would mean or probably more accurately said, would have meant if this were a distinguisable face. Essentially, a large portion of the worlds populations beliefs would have been thrown out the window, or would, at the very least, have caused a large rethinking of them.
While I agree that there are many other things that money could be spent on, I also think that there is a large necessity to explore beyond our own earth, not only to find out how/why we came about and where we might go - both here (on earth) and perhaps further.
Lastly, this is a European mission.