The issue with Rock's gameplay is that a grappler just isn't a good fit for Soul Calibur. Throws are mostly breakable here and the game revolves a lot around pokes and mix-ups and such, so a character who's whole moveset revolves around throws just plain isn't a very good idea and doesn't mesh well with the rest of the cast.
People like to say SC needs a grappler because every other fighter has one but does it really? SC is hardly your standard fighting game, we have a specific button for guard, low focus on combos, and of course the system this website is named after. If someone can adjust to the guard button they're not going to be thrown off by not having one specific playstyle they typically go for in the game.