A helo would have to be much more careful if it thought it might be shot at by a missile, so some deterrence effect. OTOH if you shot at it and missed it would active dip right on top of you and then launch some LWTs, and you'd be blasted.
If they could leave an autonomous missile pod floating just below the surface, with a submarine-like ESM mast, that would be more interesting. You couldn't get in dip range to verify subsurface contacts without being in MANPAD (BOTPAD?) range. Really the same idea as autonomous mines, which have a magnificent power of area denial.
The current tactical solution to people taking potshots at helicopters is flying in pairs. Whoever gets shot at evades. Whoever doesn't get shot at shoots at wherever the shot came from. The Navy doesn't use this tactic much because the ocean is relatively free of people hiding in apartment buildings and bushes with RPGs but they could quickly adopt it if needed at the expense of effectively halving the size of their helicopter fleet.
If they could leave an autonomous missile pod floating just below the surface, with a submarine-like ESM mast, that would be more interesting. You couldn't get in dip range to verify subsurface contacts without being in MANPAD (BOTPAD?) range. Really the same idea as autonomous mines, which have a magnificent power of area denial.