If I'm going to go CI, why should I take life passives and spec out of them later?

I feel like I'm missing something. From what I can tell the conventional wisdom says that if one is eventually going to take CI, the proper way is to take life passives, then energy shield passives, and spec out of the life passives when CI is taken at somewhere between levels 50-60. If life and energy shield are both reduced by damage, what is the point of taking life passives early instead of going straight for lots of energy shield passives?