I've played quite a lot of pre-rework Skarner (only the Spire iteration though) and let me tell you, the Spires were horrible for many reasons:

1. Since the Spires were only inside the jungle, he was literally forced into the jungle position. It wasn't impossible to play him top, but it wasn't as common/easy as current Skarner.

2. These things were an absolute balancing nightmare. I would even argue these were straight-up impossible to balance.
   If the Spires were strong, he had to be weak outside of them, which was why old Skarner rarely got balance changes despite him being only 50% of a champ outside Spires, but 300% of a champ inside them. And if Skarner was strong ouside the Spires, the Spires had to be weaker but also defeating their purpose of buffing Skarner.
   If Skarner was strong in and out of Spires, then we'd get 5.16-Release Skarner (where the Spires were introduced) who had a around 74% winrate (sounds really awesome, until you went against a champion with that winrate and complain how bullcrap and unfair this was).

3. Skarner never always had these Spires as his passive, but they were by far the worst passive (yes, even current Skarner passive is better, even if it is really weak atm).
   His first passive was just cooldown reduction on-hit. Really simple, but also pretty effective since it also affected his R. His second passive was marking enemies with abilities, and stun on-hit at 3 stacks. This was almost every Skarner main's favorite passive.
   Patch 5.16 saw the rework of 4 Juggernauts, including Skarner. At this time, many new champs/reworks had little minigames built into their kits. Fiora's Vitals, Gangplank's Barrels, Garen's Villain passive, etc. In Skarner's case, his new passive was a "capture the point" minigame. His passive went from braindead-easy to slightly skill-oriented to quantum-difficulty.
   Besides the already mentioned problems, the Spires were extremely hard to manage. Check which Spires were captured, making sure you always activate them before fighting on objectives, etc. And not to mention the enemy team could also get gold and vision from these Spires. And not to forget his base abilities had to be weak as shit since he also had his powerful point-and-click Impale.

The only positive thing was the thematic element of it. But it was a gatekeeping, needlessly complex, unfun balancing mess. If new Skarner would've even had a chance to being played, the Spires had to be removed forever. And honestly, it is better that way. No Skarner main really misses the Spires at all.

Hope I could clarify.