Reason 16% attack speed runes?

So.. many pro ADCs run rune pages like this with 16% attack speed using 4 attack speed glyphs in addition to the usual quints.

As opposed to this which only has 13.5%(14%) attack speed in favor of full scaling or flat MR glyphs.

I've been asking myself for a while now: Is +16% attack speed some kind of sweet spot or is there another reason you would take such a bad trade in terms of raw stats? I'm aware that MR is usually negligible during the laning phase bot depending on the enemy support obviously, but the same should go for 1.5% attack speed.

It just seems so strange at first glance, trading 1.5% attack speed vs 8 flat MR for example.

TL;DR: Why take 16% attack speed runes instead of 13.5%