it seems a lot of people seem to think supports add nothing at all to bottom fights and lane control.
if so, then sure the gp10 items on the support really are un important.
if that dorans blade is so vital on the adc, maybe the adc should take it easy and not go all in until he gets it or maybe the support can pick up the slack until he gets it.
point being that gp10 items become more effecient the earlier you get them, and on builds that are reliant on gp10 items for gold production and use more than one gp10 item, the earlier they get their first gp10 item the earlier they can start snowballing their gp10 process.
now practicallity and effeciency are two separate things. but from straight theory crafting perspective, its more effecient to help a support get that first gp 10 item right away.
A support is capable of performing its role nearly optimally without much items. A carry cannot.
Would getting GP10 early help as much as an AD having 10 extra AD early? No. It wouldn't.
Supports pretty much provides two things in a fight: CC and healing. CC is reliant on skills, not gold. Healing is reliant on AP, and supports get none of that.
ADCs provide one thing in a fight: damage. Damage is reliant on gold. That's it.
Think about it.