I'd argue, as you do, that using it in a limited scope makes it not very worthwhile - and that's why I'd use it everywhere. With an added pragma to allow us to warn on messages to NIL, I think it'd be quite reasonable to do that.

Consider: the objective of this is to eliminate code by making message-to-NIL safe. Adding the code to only use it certain places is essentially the same as saying if (defined $foo) and the point was to not need that.

I'd say that if you feel uncomfortable with jumping out of the plane with this chute, so to speak, this approach isn't for you. I've used it a lot in Objective-C programming and I find it really simplifies things, but much as using $_ as the default variable, either it really works for you or it doesn't work at all.