The argument that ways to provide vision in LoL are lacking is certainly valid.

The argument that Sightstone isn't needed on a support role, however, is not.

There are zero situations in which having vision on key areas of the map isn't helpful.

There are zero situations in which not buying Sightstone as a support is to the overall benefit of your team.

There are zero situations in which properly using a Sightstone is to the detriment of your team.

If you aren't buying this item, you are wasting the gold and experience given allotted to you as a support.

Buy it.