When their support has heavy CC. I think i'll go with my boots/3 pots. It actually sustains better in some ways than a doran's. You can always buiding a life scepter which is 800 gold.
So doran's is 475 vs 800g. AND you can build blood thirster. So stop crying about ADC's building boots. It actually helps them zone in and out and avoid skill shots. Oh it also helps in killing mage minions in the back of the minion pack which is dangerously close to the enemy. It's 2v2 which can turn ugly fast.
Boots are good against poke reliant ADCs like Ezreal or Cait. It's also nice for MF so you can hit Q's and run away before the enemy gets a chance to poke back. There are a lot of reasons to get boots first essentially. --- "Life is a sexually transmitted disease" -Anonymous