The simple, but not very specific, answer is when needed. The Frisco also placed steam generator equipped GP7s into E-unit consists. Seethe following photograph. http://www.frisco.org/shipit/index.php?attachments/2487-jpg.569/ This image is of No 101 at Rosedale Yard, Kansas City, KS with an airline strike swollen consist. The Kansas City Florida Special carried lots of head-end stuff, and even more so during the Christmas rush. It was one train that seemed to get a steam-line equipped B-unit more than other trains.
Tim, I know that in the past, Karl provided more insight in another thread. But in a nutshell, if the train load dictated more power than from two E units, another steam generator equipped unit FP7A or GP7 or steam line equipped unit, F3B, F7B or F9B, would be added to the consist to help get the train up to speed quicker after stops. Since the passenger trains rarely exceed the running speed of either of those freight units, the additional pulling power "out of the hole" was very beneficial to timetable schedules.