I wouldn't dismiss the SD set so quickly. No, he doesn't have Sleep Powder or Power Whip, but he has better bulk than Venusaur and the same Speed and Attack stat. It's just like pre-HGSS and post-Platinum Dragonite vs Salamence. People still used DD Dragonite when DD Mence was better in most situations, because Dragonite had perks over Mence (Namely bulk and ability to survive some Ice attacks). I would say the same thing for Meganium. A pure, offensive SD set would always be done better by Venusaur, but Meganium is worth using if you want a bulkier one. I don't know why people are saying it has many opportunities to switch in and set up, either. It has plenty of opportunities to set up. Most Pokemon Venusaur switches into, Meganium can too (Milotic, Blastoise, pretty much all bulky Waters), as well as the ability to beat RP Torterra one on one (Venusaur can, at most, put Torterra to sleep, while Meganium handily beats Torterra whatever he plans to do).