Piano instructors have different views on this topic. My intention is not to contradict another instructor - my views are mine and subject to your interpretation, but I find my approach works; otherwise, I would alter it - since my time as a coach - both live and online - is dedicated to helping each and every individual realize the potential that already exists within him or her.
You may already agree that the earlier you learn something the correct way, less time needs to be devoted to correcting errors that have become habits. All the more reason to utilize a practice approach that works best for you.
I feel that practicing certain situations, passages, etc. with "hands alone" has its place - especially when either the left or right hand is in need of executing something on the "tricky" side. However, I personally never recommend to anyone of my clients that a regular habit be made of practicing hands separately before attempting both - I feel that we, as piano players, need to train ourselves to read on both planes - vertical and horizontal - especially when it comes to sightreading.
I also believe that "chewing" a little bit of the material is better that a lot. This is really at the core of my coaching strategy - I believe practicing itself is an art - people often bite off more than they can chew during individual practice sessions, which I feel is a mistake - inquire about this if you are interested with my online coaching program.
I will repeat what I said above in another way. I am not against practicing each hands separately. But I feel this approach should be reserved for times when a particular segment of music in either hand needs undivided attention - an example of this would be a 16th or 32nd note passage that is intricate. However, I do not feel that a pianist"s mind set should automatically resort to the idea that practice hands alone is generally a better method. I believe coordination between the two hands is improved significantly by taking the opposite approach.