With 4 points invested, deadliness gives .5 dmg per level. Thats 9 extra damage at level 18. Many runes do something similar, (very small per level bonuses). Are bonuses of this nature actually worthwhile in any way? I don't see how 9 extra damage endgame is going to matter. (Exceptions made for the magic resist and cooldown reduction versions of these bonuses as their endgame final addup is easily viewed as worthwhile)
The Deadliness masteries gives 9 points of damage at character level 18. Think of it as a free Longsword
It does not seems a lot, but it represents more than a 3% AD increase on most fully geared AD carries or bruiser, as neither is nowhere near 300 AD, which is quite efficient, as far as masteries go.
To compare, assuming you go into Utility tree deep enough to grab the Greed masteries, you'd need no less than 34 minutes and 10 seconds for the Greed Masteries to pay for the free Longsword, without even taking into account inventory room and power gain through your leveling.
Not only are the Deadliness masteries efficient, but they are compulsory on attack damage characters.
AD / level runes, on the other hands, are dubious. Take either Flat AD or Armor Penetration.
What makes the interest of flat AD is gaining the upper in lane through powerful early harass and farming. It makes these last hits way easier. Here is the rune page I use Flat Armor Penetration are just deadly against squishy champions on early level because they negate completely their armor. My rune setup gives a 8% increase of damage at start and 6% at level 18, assuming your main enemy, the other AD carry doesn't buy Armor
With AD / level marks and quints , you will have 33 AD at level 18, but starting with a measly 1.27 at level 1. You will have to deal with a power level loss until level 7, which is not what you want