Oh, hey, history of Crobat. That's fun.
So, I was responsible for popularizing CB@, back a million years ago. This was at the very beginning of the ADV era, and it was a pretty simple set: Crobat @CB, Sludge Bomb/Shadow Ball/Aerial Ace/HP Ground. This was back when I was still posting under LazarusLong, approximately forever ago, playing on the old IRC bots.
I know it's an underwhelming set, completely outclassed by Aerodactyl. But here's the thing that people forget about CB@ - it was never intended to be used in the greater ADV metagame. CB@ was specifically for use prior to the release of FR/LG and Emerald, when we were still restricted to the 200 Pokemon that had been released.
This was a very different game than ADV is now, for anyone who never played in it. Most of the threats that defined the metagame (T-tar, Suicune, Celebi, Jirachi, etc.) had not been introduced yet. There were no move tutors, and a lot of the breeding-only moves were considered illegal, because there was no legal breeding combination of released Pokemon that could produce them. Hell, EVs and natures were still new, and people were still figuring out how exactly to get the most out of them.
All in all, the most defining threat from the 200 era, as I recall it, was Alakazam, of all things. Its Speed, unparalleled Special Attack and coverage made it easily the best revenge killer in the meta, and it could sweep once the opposing team was softened and the few scattered counters were removed. Take some fairly iconic members of the 200 metagame, like McGoose (McGraw's Quick Attack Silk Scarf Zangoose). The only reason it even existed was because it couldn't be revenge killed by Alakazam. I mean, hell, people were running Swellow just because it was faster than Zam.
The other really big offensive threat was Heracross. Around the same time of McGoose, people started realizing that Focus Punch on Heracross let it punch through its main counter (and the only real physical wall in the meta), Skarmory. Mixed Salamence hadn't become popular yet, and so the three most popular physical sweepers at the time, Gyarados, Salamence and Metagross, were pretty much stopped cold by Skarmory. When people started realizing that Heracross could actually punch through Skarmory thanks to Focus Punch, Heracross was everywhere. People were constantly spamming Megahorn and Focus Punch, and you needed to have something you could easily switch into those or you would lose. Even Gengar couldn't switch in, because it was always 2HKO'd by Megahorn, and even Psychic didn't OHKO in return. Basically the only thing that could reliably switch in was Weezing.
So I started running CB Crobat as an extremely niche response to the metagame at the time. It could switch into a CB Focus Punch from Heracross and only take ~40%, while Aerial Ace was a guaranteed OHKO and even just Sludge Bomb dealt around 80%. It was also a way to stop Alakazam from sweeping, since it outsped, and both Shadow Ball and Sludge Bomb OHKO'd. It could revenge kill the ever-popular Gengar, too. Actually, Gengar/Dusclops was really the only reason it ran Shadow Ball, although sometimes it was nice to have a neutral attack in case you really needed to get rid of Alakazam but the opponent also had a Metagross that might switch in. But honestly, if they had a Metagross, you pretty much had to lead with HP Ground, or else they could switch it in and set up an Agility as you switched out. Sure, you couldn't touch Skarmory, but who the fuck cares? What, were you planning on dinking away with HP Fighting or something?
It turned out to be pretty fun, especially because the 30% poison chance of Sludge Bomb meant a lot of its otherwise good switch-ins like Swampert ended up poisoned. (It also meant that Milotic was the bane of my fucking existence, but whatever.) If you didn't specifically need a status absorber (and most teams didn't - basically the only thing that had WoW was Dusclops, and fuck Dusclops - and even if they did Hera was a million times better), it was almost always a better choice than Swellow. For whatever reason, it caught on.
But then....... people kept trying to use it even after FRLG came out, which is stupid. Alakazam pretty much instantly dropped out of usage as bulkier special sweepers like Raikou rose to the forefront. Tyranitar was everywhere. Crobat's amazing speed tier was suddenly tied by two newcomers. Also, with the move tutors, Substitute was being tossed around by everyone, making revenge killers that couldn't double as late-game sweepers way less useful. With Aerodactyl out, there was basically no reason to keep running CB@.
