On top of the explanation that Mike gave (which is perfectly valid), you can also consider BoP items as a reward for having skilled up your profession to such a high level - most of the endgame BoP items are really really good.
On one hand it forces you to make choices, either try to get really nice eq via crafting professions or making money via gathering professions. On the other hand you have to carefully consider which profession you want to choose. For example I have herba and alchemy and I'm very happy with those. But often I wish I had chosen LW or even engineering (e.g. nice goggles) because of some BoP crafts. So yes, there is a choice to be made, I simply can't have everything. Some people decided to walk down a different path and got different rewards for it.