A few possibilities come to mind like if you were in a TRSA (
http://www.faa.gov/air_traffic/publi...c/atc0707.html) or if you were talking to controller who is required by LOA to provide basic radar service e.g. sequencing (
http://www.faa.gov/air_traffic/publi...c/atc0706.html). This is of course assuming you told the controller you had the traffic in sight (a lot of controllers are good and predicting the future, but we can't read minds). But more often then on not what it comes down to is nobody wants to watch two planes come together. I've learned 2nd hand that the litigation that follows a crash is horrendous. You can either say "fly heading 360 for traffic" once or spend the next 5-10 years in courtrooms and giving depositions saying "there's no separation requirement to VFR's in class D airspace".