A mix of AS and AD is better than flat AD, in the majority of situations, namely because AS is a more expensive stat to buy (gold per 1% AS compared to gold per 1 AD) than AD and your first item as ADC will, in the majority of situations again, have no AS bonuses. Plus the small bonus of something like... 6 bonus AD from quints(?) scales worse into the late game compared to AS and isn't that crippling to do without. Heck, even AS reds and AD quints works out pretty well. The mix is really based on how much early game pressure you want, as AD runes are stronger in the early game compared to AS runes, but weaker in the late game.
Depending on the champion, some people will even run full AS and no AD (namely for Vayne) and rely on just playing passive in laning to scale slightly harder in late game.