Logic, in a game? Silly!
my 2cents:
In seriousness though - I have to agree that right now sig really has no effect. With my jd and its low sig - i should be able to pop up behind someone and pop him, without aggroing his buddy 4km away, cause really he shouldnt be able to see me. Maybe instead of linking the mobs so the others aggro on you, those other mobs ability to detect lower signature ships is increased, but still a chance of them not aggroing on you.