support item build help?
I'm not sure if this has already been asked, and if so my apologizes, but could someone explain / give examples or scenarios where building the sight stone + support items would be better than just keeping them seperate. I know the obvious benefit would be to have an extra item slot but i feel like the actives on the full support items are just so useful that I myself find it hard to see when the combined item would be better. If you could also throw in some example builds and tips on how to optimize builds for different games ( because i find my self building the same items each game just in a different order depending on the situation ) that would be very much appreciated. Sorry for asking so much at once and thank you in advance for any help!
Do you want to play passive and just control the situation?
Are you wanting to provide a tiny bit of extra healing for your adc and get a shield later? 
if that is not a concern
Do I want to have a speed boost for the team for engage or disengage?
if not then
Do I feel I need a shield to protect people in an emergency?
if not 
which effects trinkets
and all of the on use items

is 3000 gold. For that much in gold difference, I could buy a Forbidden idle, and have an extra boost in CDR, mana regen, and healing power.