You're unlikely to get support for modern high speed SD cards in these sort of adapters. CF has a better shot of supporting UDMA and even if not PIO mode 4 will get you 16MB/s which will likely outperform a typical SD slot in these devices.

CF cards do wear leveling, which makes them much better suited as hard drive replacements than SD cards or USB sticks.

It's the solution for people who want to play around with old computers without having to rely on old and probably flaky hard drives.