The effect spices have depends on the specific blend.
Example: Pipeweed. Every spice mixed in this blend given you a +1 Deflection bonus to AC, and +1 fast healing. Without spices you still have the Shield AC, the Goad-like ability, and the ultimate power.
Another example: Blacktar: the stench aura is dependent on the spices mixed, and with 0 spices, the aura does not exist. Therefore, this blend needs at least one spice to work.
Some blends are designed to work well even without many spices or without spices at all, while others need a substantial investment to be useful.