Well, the answer from "received wisdom" is that maintenance often involves adding or modifying functionality, which may entail using new variables, and if there's no clear pattern for making up variable names, and you're not paying attention, ... etc.

But your point is well taken, and it's worth noting that even without "use strict", you can still declare variables with "my" to get the intended effect of scoping, just to make sure a newly added variable doesn't trounce a "legacy" variable that happens to have the same name.