That's all well and good until you want to get some real science done. The kinds of instruments carried by Curiosity are expensive and their science output cannot really be achieved by a number of cheaper alternatives. Quantity does not always compensate for quality.

Besides, in that kind of an environment it doesn't really matter how many probes you send if the electronics aren't massively rad-hardened--every one of them is pretty much guaranteed to fail sooner rather than later.