I think it is apples to apples, since as you say, the "why do you need this" question isn't relevant. Gasoline is easier to cause harm with and would be more readily accessible even if phosphorus were totally unregulated; regulating ownership of phosphorus nets a society nothing. It's as inane as banning switchblade knives when machetes, handguns, rifles, etc are all legal and available. Does anybody need a switchblade? No. But does banning them accomplish anything at all? I can't see how.

Phosphorus and gasoline even share the same potential for substance abuse. Just as you can make meth from phosphorus (apparently) you could huff gasoline. I wouldn't recommend either, but you could. Both can be used for arson, and both can be used as drugs. One you can buy anonymously in large quantities with cash in literally every neighborhood in the country.