There are small systems in which 512 MB (or MiB) isn't available, but in which some swap might conceivably be useful. Mostly networking gear, which still ships with absurdly low storage/memory allocations (my DLS router is 64 MB RAM, 8 MB flash, as an example).

Not unviable under the stock OEM firmware, but confining when switching to, say, OpenWRT. Not that I'd know about this or anything.