You know, in my honest opinion, a huge amount of controversy surrounding this issue could have been avoided if Gamefreak had the common sense to not have Substitutes be affected by evasion modifiers.
I mean think about it. A Substitute is just a decoy that sits there to sponge attacks directed at it. It doesn't move, and therefore it makes no sense for it to be able to 'evade' attacks. The only factor that should be taken into account is the initial accuracy of the move targeting it. So if any change to the mechanics were to be made, I'd much rather do this first.
But that's not the case, and we can't change it, so why bother?
In all respect, i love using BP, and i abuse it in my sandstorm team. So my opinion probably isn't going to be much, but...
Ok, firstly, BP allows a 10% chance to evade. Every 100% attack has a 1/10 chance to miss if you have a BP. Which means 9/10 times, your Pokemon faints, and 1/10 times, your pokemon survives and sweeps.
Now, that certainly looks good on paper. If you use BP, you have 1/10 chance for it to work, and win, and 9/10 for it to not work, and lose. If you want to take that chance, go ahead and use it.
Of course, the main issue here (at least, the one the TS is talking about), is combining the 10% chance with Sand Veil / Snow Cloak (which = 28% to evade), combined with a Substitute. It seems good, of course. But here are several snags.
Let us just say Garchomp. You have 3 chances to Substitute and hope that there is at least 1 miss. From the calculations by Jimbob, that means the opponent has 37.3% to hit, and 62.7% chance to screw up. So, let's say you have indeed 1 miss. That means Chomp gets 1 Swords Dance.
Now here's the problem. Wouldn't it be more effective to employ, say, SubSalac, and be ensured of getting 1 Swords Dance and a Speed Boost, and maybe even a Sub up? You still have a 20% chance to avoid attacks. So, 1 out of every 5 attacks will miss you. And you get a Speed Boost at the end. Isn't it more worth it?
Aside from that, you have Choice Band, Choice Scarf, and Life Orb (Chain Chomp) variations. If you use BrightPowder, you can't use any of these variations, and depending on situation, they might be better.
So, if you use BP Chomp, you can't use CB chomp, Swords Chomp, Chain Chomp, Scarf Chomp, and SubSalac Chomp. They all have their own uses in their own rights. For instance, a SubSalac Chomp might be better if your opponent employs 100% accuracy moves, a Chain Chomp has the surprise potential, the CB Chomp just sweeps without any set-up, and so on. Why ban just one of them?