I've got an EZGuide Plus and I use both.  GPS for mistblowing and fertilizing and the foamer for boomspraying.  The foamer's a backup for GPS if it goes down when misting/fertilizing (only happened once to me on a foggy day).  Signal strength was too low even on the top of a hill.  That'd make a fellow pull his hair out if he didn't have a backup.

  My case is a bit different because a lot of my ground is too rough (guidance lights dance back and forth because the antenna rocks back and forth) to use the GPS for guidance when boomspraying.  Might be fine with EZSteer (which I don't have), but I tried boomspraying with the foamer turned on as an accuracy check and I had no trouble being 2-4 feet off sometimes.  Following the lights was hard on the head, too.  Foamer dots with a 35' boom are easy to judge. That's terrible when I can get 0"-12" with the foamer under challenging conditions with a 25' boom (outer 5' swung inboard for rough fields).  Unlevelled blueberry ground is definitely not the same as plowed farmland though.