A lot of phrases sound odd or like ellipsis (also pointed out here, as elision) when "it" or any subject is left out. Like:
- If wrong, correct it.
- If intelligent, hire her.
- If stolen, please report it to authorities.
But for some reason, "if necessary" doesn't sound odd. As in: "I'll help you if necessary". Is there a reason, or is this ellipsis too - just something we've grown used to?