There are numerous benefits to making onion fields, ladder fields and advanced onion fields.
Portal defence benefits
The primary benefit with respect to defending portals is that overlapped fields require more links and more links provide greater attack mitigation.
Just as shields provide attack mitigation (removing a portion of any incoming attack), so does each link to a portal. Thus every extra link to a portal causes the opposing faction to have to spend more resources taking down that portal, and gives the defenders more time to recharge the resonators on that portal.
Personal / Agent benefits
There are also benefits to the individual builder. All of the extra links and fields gain you more AP and progress in your badges.
Faction / Regional benefits
For your faction as a whole and your regional statistics in particular, overlapping fields mean more Mind Units and a greater contribution to your factions position.
You have to be careful about the order in which portals are linked in order to gain the benefits though (see the tutorials linked above), and the rules do keep changing, but overall it is one of the best ways to maximise personal and faction advancement, so yes, in my opinion:
- Overlapping fields are essential to playing the game effectively.