It actually wasn't just that Oil Slick existed that got him a cameo in the show, but the fact that his design was so cool.Let me put it another way. Remember Oil Slick from Animated? The creators of that cartoon said at Botcon 2008 that they didn't even know that toy design existed until Hasbro revealed it to the public. Hasbro didn't intend for him to be in the show at all, but the shows creators later worked Oil Slick into it after they found out it existed anyway.
Also, I'm not a "show completist" and never have been, especially for Prime. I just thought about it and I've bought all of THREE Prime toys, ever: Deluxe Soundwave, Cyberverse Skyquake and Cyberverse Magnus. (There might be some other Cyberverse guys I'm forgetting, too.)
Look, Deathy--if Season 3 premieres, and the Autobots all get spikey armour in the first episode (or three), then I will gladly admit that I was wrong. Until then, let's just wait and see, hm?