Various considerations:I've got a question.
For some pokemon (e.g. Breloom), the smogon sets recommend that you pump up the the defense (impish nature, 252 HP/252 Def) but with nothing done to the special defense, or the other way round. I was wondering why this is so. Wouldn't it allow the opponent can exploit this weakness?
It may be that there are more numerous powerful special attackers which you wouldn't send the Pokemon in on anyway. Part of this comes from types - Fire and Ice are both common special attacking types, and Breloom is weak to both. You shouldn't be sending Breloom in on something that tends to run Flamethrower. Meanwhile, Rock and Ground are commonly physical attacking types, and Breloom resists them. (Comes in handy on the Swords Dance set I find)
Also, if you split your defenses, you may be unable to survive strong hits from either side. By concentrating your defenses, you take the powerful hits from one side. Meanwhile, you commonly know whether your opponent's Pokemon is likely to use physical or special attacks (Gengar won't run any physical attack other than Explosion, and Metagross isn't going to use special attacks), and can switch accordingly.
Finally, people tend to boost the better stats. Breloom's physical defense is somewhat higher than his special.
By all means, if you have reason to want different defenses, use different defenses. The Smogon sets are a good starting point, and you should be willing to alter them to fit your team.




