All the HTPC frontend's (XBMC, MP, MB) support multiple media sources so pooling is not needed. I don't know about standalone players. We should distinguish between true drive pooling and simply merging contents of different folders.
1. Drive pooling is when multiple drives/sources appear as a single source, and you can read/write to it without worrying about the physical location, and the files are balanced across drives as needed.
2. And then you have something like Libraries in Windows 7, junctions/symlinks, multiple paths in a XBMC source etc. All these will let you see all the content as 1 folder, but it doesn't actually manage the data for you or handle drives getting full.
For most people #2 is more than enough I think.