I'm trying to think of a valid analogy here but I apologize if this doesn't make a whole lot of sense due to how late it is and as usual, that means I'm going to use a Magic: The Gathering analogy. :P
Let's say you have a 3/3 creature out on the table compared to a 2/2 creature that gains +2/+2 for every consecutive turn he hasn't attacked and loses them all when he attacks.
On the one hand, the 3/3 creature looks better in every respect - until you are in a situation where you don't or can't attack.
However, in the other case, the 2/2 creature that grows is just continuously better if it's a situation where you don't or can't attack.
This is essentially the core dichotomy of Sightstone versus Heart of Gold - While the Sightstone is tremendously gold efficient, it is only generating gold if you are actively using and aggressively warding on cooldown / base backs.
While Heart of Gold is generating advantage for you no matter what you do, Sightstone forces you to take additional actions to translate to gold efficiency. If you only use Sightstone as a replacement ward for your lane - it's not very gold efficient at all compared to buying 3 or 4 wards for example.
Sorry, that was terrible.
TL;DR - disliked the passive advantage that HoG was giving from taking inaction - while Sightstone doesn't actually generate gold top-lane unless used aggressively to ward more than just the top lane.