Hydra has a defensive boost in lifesteal sustain.
Scimitar is overall the AD equivalent of hourglass. Although it's cost would use some help. Removing some of the strongest abilities in the game that Hourglass only delays.
The example I gave BotRK was a defensive one. Taking 15% Hp off a tank and healing yourself is a quite a bit if you don't build HP. Topped off with a MS buff/slow.
Hourlgass allows you to avoid attacks if timed right and maybe your team can save you form a sticky spot. That mostly relies on the team being there to help you. Still have to time the item right. If you're CC'd you can't even use it.
It's a great but situational item. Reason it's gotten so much is that most games are lost from your own mistakes. Higher MMR/pro players get it so often for this reason. The high AP(which got nerfed) is there to keep it's cost high. Unless you want it available to a wider champion pool.