[jacquesm]

It would also serve as a warning that we're only 'mostly harmless' so if a house-call is in order (or a preventive visit) then better come packing heat.

Of course that assumes a whole raft of things such as the ability of ET to show up in person but since we don't know anything about the technological state of the rest of the universe mostly because we don't know anything about them at all that's not too far out.

[rbanffy]

To any civilization capable of showing up face to face, we are completely harmless. They don't even need to bring the guns: just pointing the engine our way while braking would do the trick.