I have always found it useful, when that unanticipated broadside comes up, or that ship you werent attacking suddenly sneaks up on you.
The number of skill points changes its duration, not its strength as best I can tell.
I think its usefullness is a matter of timing, especially if you only have 1 point on it, as its duration is somewhat short. It does not appear to make your ship invincible for that short period of time, but damage taken is reduced for that time period.
Zeppers lvl36
http://pirates.teamofx.us