A lot of adcs, especially hypercarries, would like to use a keystone that enhances their dps. To cover this use case, lethal tempo was invented.
Lethals tempo's mode of operation is very unsatisfying in the current state. It really feels bad if you suffer a loss of 80% attack speed in the middle of an extended fight. Also, it disencourages you to harass enemy champions late game because you know a teamfight could start 5 seconds later, when you're already going on cooldown.
I think lethal tempo should be the rune for extended fights. There should be no cooldown at all on it, instead, it should give its user ramping attack speed as long as he stays in combat, capped at some reasonable limit. It could provide its user with (base value * 1/3) additional attack speed for the next auto attack, that means you would get the full value of the keystone from your 4rd autoattack on (including 4th aa as well). The base value could be something like 30% at level 1 to 60% at level 18.