Seem to be in the minority, but I loved Kaleidoscope. I always wondered if it might be because I got in to the band a little later.
I got into Transatlantic in 2005, just a few years after Neal left both TA and SB, so I latched on to SMPT:e and BAF. When The Whirlwind was announced, I was ecstatic, and eventually loved it. With Kaleidoscope, I was excited as well, but not as much as TW, but in the end, I really love the album as well. Do I love it more than the first three? No...but I enjoy it for what it is and find myself spinning it as often as any of the other three albums. I think I listen to it more often now because it's still newer than the others, which I've listened to hundreds of times, so Kaleidoscope is still "fresh" to me.
As for their contributions, I'd say while it wasn't much new for TA, it was still a bit refreshing to hear compared to what the others were doing at the time. It came out before the first Neal Morse Band album, and I felt like Neal's Momentum was a bit underwhelming, especially after Testimony 2, so hearing Kaleidoscope from the Neal-fan perspective was nice to hear.
As for Roine, he had just come off a two-album reunion with The Flower Kings, so this felt like a continuation of his 2012 and 2013 TFK releases, but little did we know, Desolation Rose was to be his last TFK album, and to be honest, like others have said, his material since Kaleidoscope has been spotty at best (Anderson/Stolt was more Anderson than Stolt than I'd hope for; The Sea Within, while good, hasn't had much staying power with me; and Manifesto has come and gone in my mind and hasn't made much of an impact for me yet). I think working with Neal, Pete, and Mike brings out some of the best in Roine, as I'd take The Whirlwind/Kaleidoscope over anything he's done in the last 10 years.
And as for Pete, well he did contribute one of the best themes in "Stranger In Your Soul", and some tasty bass playing and themes in The Whirlwind. His contributions in Kaleidoscope were probably not as huge, though I remember that middle section in the title track, where his singing sounds a BIT weird, but it was cool to hear him sing on the album. I always think that, while Marillion is always seemingly "preparing for a new album", Pete surely has a lot of material stored away that he keeps demos of, though he did release a new Kino album last year, so who knows how much material he could bring to the table. Of him, Neal and Roine, however, he always seemed to bring the least amount of demos into the TA sessions anyway.
Would I be excited for new TA? Heck yeah! Would I be excited if they just toured! Of course! Would I be sad if they never did anything ever again? Definitely, but I'm happy with what we've gotten from them. I do agree, though, that I hope they don't reunite under some shared sense of obligation to release an album, and that if they do, it happens organically.
-Marc.