jbtc10:
First, I don't know what you mean by "wasted" stat points. That implies that Mega Latios would be better off without the extra Atk and SpA, which is not true. Any lower and LO Latios's power advantage would be even more significant than it is now. It might not be enough to overshadow LO Latios in terms of power (and maybe some of those Atk points could have gone to Spe), but they aren't wasted.
Second, I think you misunderstand what I'm arguing. I'm not saying that Mega Latios is a lot better than its regular form, certainly not to the extent of other top Megas. However, it is better overall. You can downplay the advantages it has all you want, but they still exist. And no, the extra Def/SpD is not "lame," nor are the other defensive advantages (lack of Life Orb recoil and lesser Knock Off damage). Seeing as how one of the main reasons that Latios is so popular on offensive teams is because it offers a decent defensive backbone with its solid bulk, good resistances, and reliable recovery, additional bulk and longevity is actually a pretty significant advantage.
Now, I know that you can't look at Pokemon in a vacuum, but that's not what I'm arguing either. If I were looking at Mega Latios purely in a vacuum, I would be nominating it to A+ rank easy, maybe even S. What I am instead saying is that restricting Mega Latios to the B ranks should be enough to get the point across about his opportunity cost without the need to drop him lower within those ranks. While the opportunity cost associated with Mega Latios is high, I don't think it's enough to drop what could potentially be a top-ranked Mega all the way down to B-. I think as long as it's anywhere in the B's, it's good enough, and I think it should be high in those ranks because it's simply better than the other Pokemon among them.
It's fine if you disagree with this approach, but I just want you to understand what exactly I'm arguing. I'm honestly just throwing out an opinion here because I don't think there's a clear-cut answer to this, nor is there really much of a precedent to work off of. I don't think we've ever had a Pokemon that was so good on its own yet let down by such a huge opportunity cost as this. This is one reason that I brought up the question earlier in this thread about how much we need to weigh opportunity cost, and it's cases like this where your answer to that question is going to have the most impact.