Several objections have been raised on the talk page in the past, but I suspect that much of it stems from mappers’ unfamiliarity with this kind of space from their own childhood, which led to misleading documentation at times. Every school is slightly different, even in the same region and at the same grade levels. I happened to attend one elementary school that had an easily identifiable space of this kind, next to the playground, and later another school in the same town that did not have one.
It doesn’t help that “schoolyard” is a term that in some dialects of English can also refer to the overall landuse=education. For this reason, I personally tend to refer to one of these spaces as a “blacktop” in conversation. But that’s a slang term that’s even less suitable for a tag value. (Most of the blacktops I’ve mapped are far from black. What school can afford to keep it in such good condition?)