I had a conversation with DarkSlay about this on IRC a few days ago. I'm going to throw out some generalizations that arose from that conversation. I obviously think this stuff is spot on, but I'll leave it up for your interpretation.
Weak Armor allows us to explore late-game risk. For the most part, you aren't going to be using CAP4 with Weak Armor early on in the game. It simply is going to have too many checks to be of much use. If you do use CAP4 early, you'll get the boost, but your weakened defenses leave you as easy pickings for Scizor, priority, and Scarf-mons. And honestly, if you're running Scarf Weak Armor CAP4, then I don't know what's up with your strategy, haha. In general, you're going to wait until the end of the game to pick up the boost and sweep when your counters are removed. It tests late-game risk.
Illusion, on the other hand, lets us test out early-game risk. That ability is going to be used early-game, for the most part, due to its weakness to hazards. Illusion allows us to test out the concept of lures, which is something we've lost a little bit of in the transition to Gen5, in my opinion. With just Weak Armor, we won't be testing out early-game risk. That is why Illusion is the absolutely best complementary ability. It doesn't function very well late-game, but it works as an excellent lure at the start of the match. Illusion would allow us to check out risk at the beginning stages of a match.
Overall, Illusion and Weak Armor complement each other very well. One lets us test early-game risk, while our current ability will be working with late-game risk. I think that these two abilities would absolutely work best in tandem with one another to give CAP4 a plethora of risky situations to test out.