MM2 has a full weakness cycle, it's just that Metal Blade is way too powerful. People also tend to play on Normal difficulty where everything deals 2x damage, so Metal Blade just rips through the whole game. On Difficult, I use pretty much all the regular weaknesses (except maybe vs. Flash).
MM3 is weird, I don't think the two cycles make sense at all. If they wanted us to beat two bosses with the buster, they wouldn't place both of them in the same cycle (buster deals 2 damage to Magnet and Top).
I use Shadow Blade against Magnet, Gemini and Spark (plus Shadow himself at the refight).
You guys need to watch some Street Fighter III combo videos. Aegis being Ryu's weakness will start to make perfect sense.
This may be the most compelling argument so far! XD
Actually, its not. I can link more combo videos where Ryu is HELPED by Aegis Reflector than he can link combo videos where Ryu is destroyed by Aegis reflector in the same way its used as his weakness in SFxMM.