Essentially the reason melee sucks is because you're required to stand closer to your enemy than magic or ranged. I'm not saying you HAVE to be right up in their face, just that you have to be CLOSER with melee (barring light discs which are garbage due to low damage and no prefixes) than with ranged or magic.

What that means is that dodging enemy attacks is harder, and even if you DO dodge them, you're very likely to move out of the zone where you can hit your enemy anyway. The solution, as evidenced by the many people who think the warding prefix is the coolest thing since sliced bread, is to stack up on defense. However, a much more effective solution is to use ranged or magic damage and just learn to dodge. This allows you to use the preferable prefix of menacing, dealing a grand total of 20% more damage to the enemy.

Basically you want your setup to be as offensively geared as you can. This is because extra defense only matters if you can't survive without it. Whether you beat your enemy with 400 HP left or you beat him with just 3, you won and he lost. In a game like Terraria, how many hits you take is largely based on strategy/skill, and thus learning to dodge, as well as keeping away from your enemy, can greatly lower the amount of defense you need to win a battle.

Offense, however, works slightly differently. Extra offensive power helps to quicken your victory until you can simply walk up to an enemy and one-shot him, which will never happen with bosses in Terraria. Therefore, extra offense will never be redundant, and will definitely never be detrimental to your effectiveness.