Ehhhh. I think it's more situational than that.
If you're a support that initiates, roams, or can at least survive while wandering, yeah, a SS is a good purchase. But if you're a squishy, hug-your-team support where roaming is an all-expenses-paid express ticket back to the fountain, then a sightstone doesn't make as much sense as some of the other gold income item upgrades.
Take Morgana, for example. Getting the sightstone upgrade for FQC makes a lot of sense, because while you do lose the slow, you can actually survive going to ward objectives and the like even if you happen to run into the enemy jungler or a roaming assassin. If you're on Soraka, on the other hand, then getting within warding distance of a bush by yourself can be lethal - and the utility offered by the group speed boost can be very powerful to help engage or disengage, not to mention the vastly improved itemization.