I believe strongly in snowballing botlane very hard, very early. I set up my runes and masteries around this goal, and I choose my champion pool around it too---and encourage my ADC to pick in order to do this as well. If I can beat you in an allin right now, I'm going to allin you right now. And most of the time, I can allin beat you in an allin right now. Why? Because you didn't set up for the early game. You're running gp10 quints as opposed to my flat HP or armor or MR. And then, when laning started, I was willing to drop myself to 50% HP and then immediately burn a pot while I got you to 75% HP so that once my pot brings me up to 90% I can kill you.
You beat me in lane by having your jungler come bot early, but I warded early to make sure that I wouldn't die, and if your jungler stays bot then the rest of my team can snowball early---that's fine with me, I'll just start playing passive now.
I keep beating you after the first return to base, too, even if my carry and I were unable to get a kill. Why? Because you bought a philostone or a sightstone, and I got a red crystal and cloth armor or a chalice. I used my money as efficiently as possible, and you didn't, so now I'm going to force a fight and my carry and I will kill you. Now your GP10 cost you a kill and an assist's worth of gold.
That's why I hated sightstone on release. I thought it was just another trap for supports to sink their gold into early, instead of buying to win. I bought one in my first handful of games on the new patch and then stopped building it for a few weeks.
Then about a week ago, I decided to start building it the instant laning was over. My first complaints about it as a post-laning item was that it takes up too many item slots, since 3 wards isn't enough on the map at a time, so I need additional green wards and a pink ward or two, and three slots for wards is too many. But after several games with a sightstone, I've just started making my team buy pink wards for objectives while I use two slots on wards---one for sightstone and one for a stack of 2-3 green wards. With this setup, I place wards constantly. If it's an important ward that I won't be able to replace any time soon, it gets a "permanent" (non-sightstone) ward; otherwise I just place a sightstone ward, knowing that I'll likely end up replacing it within a minute or two. I go back to base very fequently to refresh the sightstone stacks and just don't worry about using them up quickly. This changing vision is even more useful than a smaller total set of permanent vision that's more spread out around the map.
What makes sightstone so strong is not just that it's a gp10 that saves you from buying three wards every three minutes. It's that it lets you ward constantly as your team moves and old wards become unnecessary. If you sort of wanted to place a ward before, you can definitely place one now.
But sightstone is not a laning item. Its massive advantage comes from the fact that you can place a number of wards with it that would be considered 'overwarding' if you didn't have it. In lane, this advantage isn't all that helpful; it's actually only giving you the two wards at a time, since during laning your map position is fairly static. So your choices are:
Sightstone --- 700 gold, 2 wards, 100 HP
Ruby Crystal and two wards and two potions --- 695 gold, 2 wards, 180 HP, 300 Sustain HP
The first time you go back to base, if you even have 700 gold, you're sacrificing 80 HP and two potions in order to save 150 gold later. Consider whether this is worth it when you're playing a ranged support vs a ranged support and the extra sustain and HP can allow you to win the 2v2 to the point of getting a kill. Or if you're playing against a kill lane and the extra HP can actually save your life. A death makes your sightstone cost effectively 1000 gold instead of 700---or even more if the enemy got an assist off it.
Furthermore, you frequently don't have 700 gold a single time you go back to base as support during laning, and so you actually have to save gold and not make a purchase of stats one time that you're returning from well. This loss makes you even more vulnerable to being killed early.
When do you get a sightstone when you're laning? If you have no kill potential and are playing against a lane that has no kill potential, or if your lane is so far ahead that it doesn't matter what items you have, you're still going to be able to win. This could be because you're Leona fighting a Sona or it could be because you're a Sona fighting a Nami but your ADC is four kills up on theirs. Just remember that you are an extremely relevant member of the team and a viable target for focus during laning when you account for 50% of your team, and if you give up too many stats to save gold for later, you can easily allow the enemy to snowball past you.