Crest regen knight, moonlight warrior, blessed acolyte, carbuncle, combat monk, haste soldier, magic doll, these are some of the foundation units of their spheres. I think they pretty much gave out the foundation parts of the puzzle so you can expand on the other spheres much easier like zanador said. When it comes to alteil, I don't think anyone plays one sphere only because 25 card files really limit what you can use. Everyone who plays this game has more than 1 color at least. At first I went through the state of "keeping only gowen units" too, but then I got punishment hole in a treasure duel, so I thought "maybe I should try adding some lawtia multisphere. Then the amazing thing was I got proxy of soul from a treasure duel, and that officially dragged me into something else other than gowen. From there I started collecting other spheres and built my 2nd deck which was a lycan rush. Then I was interested in refess because of the buffing, and I built a guardian deck, and snaf, femiel, dub rido leviathan, all those awesome cards gave me the urge to step into falkow as well.
Without these extra cards, I never would've thought to build a multisphere lawtia/gowen deck because I'd lack the solid lawtia units, but having moonlight warrior, crest regen knight and magic doll, I was able to combine the solid defense of lawtia lvl 3s and the gowen speed and power together with punishment hole and proxy I got, and made a decent file with it all.