There are numerous benefits to making onion fields and ladder fields.
Firstly there are the obvious benefits to the individual builder. All of the extra links and fields, gain you more AP.
For the owners of those portals there are the benefits of extra attack mitigation from the extra links - every extra link to a portal causes the opposing faction to have to spend more resources taking down that portal.
For your faction as a whole and your regional statistics in particular, the 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, and the rules do keep changing, but overall it is one of the best ways to maximise personal and faction advancement, so yes, it is more than worthwhile.