I think if a pilot has the services available, then they should use them. In San Diego, the airspace is congested and given the size of most aircraft (small) flying around, it does not hurt to have someone with a radar watching out for traffic as well. Additionally, if you are a student pilot, using ATC services presents the opportunity to practice that sometimes challenging "pilot speak". Practice makes perfect.
I fully agree with talking to ATC even when passing outside of their airspace - it keeps everyone in the loop of whos doing what. What experience have people had with ATC in different continents - such as Europe and the USA?