User blog comment:Lokiwg/Runes Lets talk about them!/@comment-5266525-20140902233630

Level 6 really isn't as early as you'd think. The outcome of a lane can be decided earlier than that especially if you happen to be in a lane with large early power spikes at levels 2 and 3 (eg. Leona and Graves). That goes for just about any flat rune vs its scaling counterpart, and that's not considering the junglers who need every scrap of help they can get to survive their clearing.

As a general rule for armor penetration marks, they're more oriented towards the lategame and for very aggressive laners. You'll generally have a much harder time performing normal laning duties like lasthitting with them than with something like flat AD.