Are Penetration runes generally worth it?
I started playing again after about half a year or so and want to get serious about playing ranked. I used to run a small repetoire of rune sets with either magic or armour penetration runes and the remainder geared towards armour or magic resist. I have been looking at builds online and most people don't maximize their penetration runes, but instead go for flat AP/AD and often get health runes or attack speed. I always assumed that because few items had penetration, taking them on runes was a good idea - you can get up to ~20 of either one right off the bat.
Could someone more experienced than me explain why flat AD/AP/AS/Health tends to be more common in well-rated champion guides?