Silly, silly Matt.. don't you know that as soon as Space Marines turn traitor they refuse to touch tech that isn't at least 10,000 years old? Those Plasma cannons are for those disrespectful young Asartes with their Land Raider Crusaders and their rock and Roll and their Ipods and their Thunderhawk Gunships and their Britney Spears.... But to answer your question, no, we don't get Plasma Cannons except melded into an Obliterator. Just like we don't get Land Speeders or crazy alternative pattern land raiders or Thunder Hammers. back in MY day we didn't even get Bolters! We had a rock, and we had to take turns throwing the rock....