I personally prefer leaving the measurement in the instructions for two reasons.

1. I'll often use the ingredients list (and quantities there) before cooking to ensure that I have everything I need ahead of time. Depending on what ingredient it is, I might not mise en place it. In those cases, a step that says "add ingredient" would require me to go reference the list in the beginning, losing a bit of context.

2. It's not often, but I've followed a few recipes that require a particular ingredient in 2 different steps in different amounts.