a lot of marksman adcs are squishy and have have 0 mobility, if not all, actually a lot of bot lane champs have 0 mobility, and if you are using a marksman adc then my argument is not to take ignite, i dont play ranked i play only draft but i've found my survivability to be higher when swapping ignite with ghost or heal, anything that gives movement speed to get you out of bad situations.

plus when playing a marksman for adc, it's just so unlikely you'll b using ignite in bot lane because you'll have to get in range to use it which means when you're gonna be getting dove, you'll use it and PROBABLY get a kill, but at the cost of your life also, it's just a stupid trade and I think it's better to get away by using a different summoner spell, your AA has a bigger range than ignite

the rune reasons are again for slightly better survivability and damage