"Rejects" is a pretty strong word, isn't it? I can see it for what ended up as the Sonic 4 Boss song, since it indeed sounds like ass, but to my untrained ears this Knuckles Bonus Stage theme aka Adventure's Twinkle Park is pretty damn catchy. I can't imagine that the reason why it didn't make the cut was because it was deemed inferior. I think Senoue feeling like not enough people heard these due to the lack of a 3D Blast Japanese release gives a pretty satisfactory explanation for the "high" reuse incidence of this game. I would also believe it if these tapes were originally composed for something else and the fact that they made it to both to 3D Blast and to later titles was just the result of Senoue frantically looking for places to put his scrapped work to use somewhere, but who knows, there you can only speculate. It's not impossible that these were scrapped Sonic 3 tracks, but it can also be explained by a bunch of other things, there's not a whole lot of evidence for it so far.
Jun Senoue was hired by Sega in 1993 - it's not entirely outside the realms of possibility, but it seems a bit... unlikely that they'd have him score the entire soundtrack of Sonic 3, one of Sega's most important releases within his first year. Even if he'd been composing and playing music all his life, there's a learning curve when converting this to something that'll run on the Mega Drive.
I don't find it very likely that he was ever supposed to compose the whole of Sonic 3, but there is this quote from Iizuka that outlines the music selection process of Sonic 3&K: Given this, it seems there's a good chance that Senoue composed some tracks that were intended for use in Sonic 3, but didn't get picked for the final soundtrack. Whether or not those tracks were reused for other projects we'll most likely never know
This reminds me of that time it was discovered the Special Stage music from Sonic 3 was made earlier (namely, the SegaSonic Bros. prototype from 1991/92. The music used for Levels 40 to 49). Maybe it was also the case that Jun Senoue was hired to create stock music with particular themes. Then when the music was recorded, it was added to a repertoire of music. Then game producers could listen to the library of music and choose was sounded good for their game. And then later add the composer to the credits without having to have asked the composer to make music from scratch.
Listening to Casino Street again, I can definitely hear the similarities, but that sounds to me more like a send-up of the Spring Stadium theme than a direct copy of it.
Much as I'd like to agree, Sega's done similar before. For Phantasy Star III, they asked Izuho Numata, who had only done a few small games and who had never touched RPG music before, to do just that. This was also for what was the third game in one of Sega's more popular IPs, she was doing it completely alone, and she was completely terrified. She did it anyway, and in a big way: Phantasy Star III has way more music than the previous games, repeatedly "upgrades" the world map theme as the game progresses (a very rare aesthetic choice), and even has different battle themes for if you're winning/losing/neutral (also extremely rare). No surprise they brought her back for the next Phantasy Star, even though it was made quite a few years after. Fun fact: she was the one who did that Ooze final boss theme everyone likes. Likewise, I'm pretty sure Phantasy Star Online was Hideaki Kobayashi's first game ever. We all know how that turned out. That man is worshipped for his music. Similarly, Sonic Adventure 2 was basically Tomoya Ohtani's second real game (after, uh, Chu Chu Rocket, of all things), Adventure 1 was Kenichi Tokoi's first real thing after bits and pieces in a few previous games, so on and so forth. These are all collaborations, yes, but even having "nobodies" do multiple songs for a major project is a big deal. I remember a review with Senoue where he states he reused Sonic 3D music for Adventure 1 because the version of the game with his music hadn't been released in Japan (at that point neither version was, funny) and he just really wanted to get this material into a game Japan would actually see. Something similar probably happened with Sonic 4: he didn't like that the boss theme was unused all this time, so he decided to use it. The entirety of SegaSonic Bros. was similarly "unused", and the composer probably wasn't too happy about it.
He said that during Summer of Sonic 2011 in an interview with (I believe?) SegaNerds. The video's gone, however, as I unfortunately discovered when writing the Sonic Adventure Wikipedia page. It was deleted on YouTube at some point, and the Internet Archive doesn't have it saved.
Guys, it's not closer to Spring Stadium, it is exactly the same melody that's used by Spring Stadium! I don't know how anyone could mistake it for another track, it is literally the same song.
Yeah, it's much more of a circus or clown sounding theme than a boss theme. It's completely goofy and not in the slightest but intimidating. Sonic 2 did a much better job at making the boss theme sound equal parts silly and menacing, which Mania managed again several years later. Even SA2's E.G.G.M.A.N. does the job in a different way. I can see what Senoue was going for, but it didn't work. And he didn't need to bring it back for Sonic 4. It was a nice touch on paper, but... No. There's a reason that Travellers Tales went for a different boss theme. I do find it interesting that both the S4E1 boss theme and White Park from S4E2 originated as unused from S3D. Obviously White Park 2 was previously used for Twinkle Park's Pleasure Castle, but the fact that Senoue went back to his final MD game for S4 is quite cool. I think the guys heart was in the right place for S4, even if the direction was way off.
Also does confirm officially that it was intended for use as a Boss track in Sonic 3D - while the track was found in the ROM, I don't think it's ever been explicitly pegged as for a boss before (even counting the Sonic 4 usage for that).