Last whisper becomes gold efficient against normal resistance targets (50 Armor) if you have around 180 AD. Now obviously the Percentage Armor Pen is exceptionally strong if you are playing against a Tanky, High-Armor Champion (or even Team).
Armor is usually easier to build than MR so people don't hesitate to buy it. Also a lot of AD Casters either play top or roam there pretty often which allows for more effective trades/ganks in terms of damage.
It's usually not recommended to buy LW early on when playing Talon vs Ahri if she isn't building any Armor and if you're not ganking but If there is a Nasus top who needs a shutdown for example you can rush LW and try to deny him as much as possible.
Lolwiki has a pretty nice Curve about the Gold Efficiency on LW.