Spellbinder's true potential is unlocked with Rabadon's, when the extra AP you get for a few seconds also gets boosted by 35%. It's a rather niche purchase but not "unwanted". It's limited, given that its effect also gives you % MS (based on charges) to possibly chase down your opponents but the thing is that it mostly functions like a makeshift Luden's.
It's an item that boosts spells, not one that is focused on some form of defense (Zhonya's effect or HP). Hense, mages would always skip it because it's not an early purchase and later, you have to itemize against the enemy items (Void Staff, Rylai's, Liandry's, etc). There's simply not a lot of wiggle room because mage's purchases are mostly predetermined by the matchup and the eventual late game.
It's not a trap purchase (like old Zephyr was) but it's not a staple purchase either.
Now, it could be a great support item (as if Mejai's wasn't) but the cost is a bit off-putting, given that it's full price is tailored for a solo laner (2900 gold). It functions well against teams that spam a lot of spells or even for your own team, if there are champs like Cass or Kass around. That would charge it up quickly to 100 stacks.
If i were to buff it, i would likely increase its active duration to 6 seconds (from 4) or reduce the active CD time to 45 seconds (from 60). Or, maybe allow it to charge up to 200 stacks but then ignore my previous buffs. Maybe time gate this +100% overcharge to make sure that mages can't just sit on it and threaten an immediate 100-0 burst, which means that likely in the mid-late game, this item would be charging from 150-200. So no 200% charge early into the game for easy AP nuke damage. This would allow a semblance of balance around it and make it better as a standalone item.
So at the first 10 minutes, Spellbinder is hard capped to 100% charge. After 10 minutes, every 1 minute (or 1.5), it gains additional 10% overcharge. That would mean tha at 20-25 minutes, it can reach its full potential of 200%. Rod of Ages has the same mechanic built in so it's not a stretch to do something like this. We can call this effect "Arcane Capacitor" or "Arcane Battery" or something along those lines.
I think it's a great item overall but a little bit constrained in terms of its usability. Luden's is 300g more expensive and has 30 AP less but it helps a lot more in waveclear or poke because its charge mechanic is passive. Spellbinder is also not immediately usable because you need to stack it up first and it's mostly going to be used on a teamfight/1v1 instead of waveclearing.
on any champs.
What make me thinking about it is the duration (4s).
Everytime I activeted it I barely have the time to use only one spell rotation (since spell are gated to CD).
Comparing it to gargoyle stoneplate and essence reaver (time duration gate effect) gargoyle last for 4 second but even if you have all on CD u still be tanky and Essence reaver rely on Aa which are only gated by attackspeed and last 6s after using one auto.
I think that since is CD gated (with the few exception of ap Aa dmg dealer like teemo or kaisa), the duration should be up and the ap gain should be low.
Tough?
because they have CDR, AP, Move speed, Activateables that are useful, and are generally CHEAPER.
gets little play now. 
are all just far more useful.
who generally lean toward 
vs 
for stacking it
whenever i get the Money.
My main Problem is that its almost impossible to get semi-important Items anymore because the Game ends when most haven´t even build their Core Item(s) yet. Might also be the case for Spellbinder, since it isnt a Core item on anyone.
and 
are kinda mandatory.
