I don't know social work... but I did go straight from undergraduate degree into a Masters/PhD program, and based on this I think the entire graduate experience which covered many abstract topics would have meant more to me if I'd first spent time in the field. For me, this would mean in industry. For you, this might mean time spent working as (or with) a social worker.
Higher education is also higher level education, and will mean more if it is building on something you've seen with your own eyes and not just in text books. Being able to relate the things you are studying in a Ph.D program to personal experiences will be a lot more rewarding than just studying something in the abstract. It will also help you pick a dissertation topic that you have personal knowledge of and not just academic knowledge of.
Having a year or two between masters and Ph.D spent working could add meaning to your Ph.D.