The reason attack speed runes are bad on Tristana is because she has a buff to increase her attack speed by a pre-set percent. You would get things like Bloodthirster, Frozen Mallet, the works. But if you play someone like....Teemo, attack speed would be more beneficial. His poison is there to do extra damage (35 a second, I believe). Meaning with more attack speed, you have something backing you up. He has nothing to aid him in attack speed.
Blind Dart, Move Quick (increased movement speed), Passive Poison, and Mushrooms.
Unlike Yi who can just use Beserker's Greaves and Last Whisper. He only needs to use Wuju + Highlander to reach the cap and kill anyone in two seconds.
Or Twitch. He has increased attack speed when coming out of his stealth AND from his ultimate.
I also remember seeing a very good statement in another thread.
You want to use good attack speed if the enemy has a lot of tanks. More attack speed will whittle tanks down faster. If their team has squishies, it would be more advised to use attack damage. So you can kill them before they kill you.
Of course, with a rune book, you never know. Some people fill their book with only one type of rune, some balance it out.
Personally, with most melee DPS, I would
go with 9 Red Armor Pen, 9 Yellow Critical Damage, 9 Blue Critical Damage, 3 Purple Critical Damage/Experience gain.
Simply because armor penetration is awesome. Makes those high armor users (like Rammus and Alistar) pay for stacking armor.
EDIT: If you do decide to use attack speed, things like Sword of the Divine, Black Cleaver, and Madred's Bloodrazor is godly.