Mathematical equations representing (approximate) solutions to physics problems are patentable, then? Let's turn the clocks back to 1900 and figure out where we'd be technologically if every such "invention" had a 20-year period of exclusivity.
Edit: An early example of a mechanical "algorithm" patent - http://www.google.com/patents/US3765263
The premise of your argument relies on the vague notion of what's a "practical" problem and what's not. I don't think that's any more tenable than any other defense of the current patent system I've ever seen. It's all vague, and exchanges like this are pushing me to a more extreme position that all patents are bogus, rather than granting that some "math" patents are connected to specific practical problems and thus patentable.