You let them hit your shield, you got a free hit no matter who it is.
You put up that shield and I'm gonna come at you with a grab. Where's your shield now?
That's right, it's in my hands.
On another note, when I used to play Brawl, some of my friends beat me with Ganon, and it really got on my nerves because he's not generally someone I should've been losing to. Naturally, I was an aggressive player, and back then, I was almost always the one to approach, and sometimes lead me to my doom of running into a smash attack. It was very frustrating, and I got angry at my friend for spamming Side Special with Ganon. (Which is a command grab, which means it goes through shields)
After this, I eventually improved as a player, and realized aggression is viable (if not better) if it is executed properly. It's a risky game, but you can also force your opponent to play your game by pressuring them, and executing this pressure properly. This makes it a rewarding play style.
Although in the case of the Ganon issue above, I just learned to strafe back in case he'd do a Side Special, thus baiting and capitalizing on his mistake. By faking aggression, I caused him to throw something out, and I punished it.
That's really what my playstyle revolves around, aggressive baiting. To put it simply, I play half aggressive and half bait. (The ratio's are most likely NOT exact) I can run up to you, jump back, in an attempt to make you throw out an attack, and punish it, or the other time I'll just run up and grab you.
(Especially if you like turtling in shield. That just makes it easier.) I can run up and shield, and the moment that frame or your attack tacks my shield, I throw out a quick jab, grab, or some other move to interrupt you.
If anything, defense is probably the easiest to execute against an
incompetent aggressive player. Aggression is probably the hardest play styles to master, considering the amount of players who complain about defense, but it's very rewarding, and defense has no advantage over it other than being easier for
scrubs.