The way I see it, upgrading it only gives an extra ward and mediocre stats, when that gold can be spent on something more impactful. Am I missing something? Or is that extra ward worth that much?
I just want to clarify that I'm not trying to say it's worthless to upgrade or anything. I'm just genuinely confused why so many players I see do it.
You know those charges you use to get gold. aka for the Relic shield part
, you get 2 charges to last hit a minion under certain amount of health. And they recharge pretty slow. Then you go Targon's Brace, you get more charges(one extra if I remember) and they recharge quicker. Same will happen as you fully upgraded it. I'm not sure about Ancient Coin, but I think this applies to Spellthief.
So if you are able to get more charges and quicker, you can get more gold, since it is your source of income.
Also, when you fully upgraded it, it gives 10% cooldown reduction.
You aren't really supposed to fully upgrade it until the midgame (around 15-20 minutes in)
That's why they gave that option, because they want people to buy the first upgrade then be able to move onto other items without having to worry about wasting a huge chunk of gold on just a sightstone then taking forever to get your other supporting items, exactly like you said
They might just do it because they don't know better.
should be upgraded as soon as possible since it gives it an extra charge.
and 
can both wait.
