Rolls Royce was a strange one, underlined by the record result. Arnold made more sense when they were playing one or two levels above where they are now and IIRC we were sharing their facilities at the time Spencer got that injury, so it was an obvious friendly to arrange - if a highly regrettable one with hindsight.
I think Notts78 is right though, the Trew years seemed to demonstrate that prestige friendlies were not helpful in terms of preparing for the season ahead. Ideally you want to play a couple of away games to sides a level below and a few at home to sides one level above. Curiously, as late as 1969 we didn't play any pre-season friendlies at all (and that was the case for most 1960s seasons). It's the point Sirrel arrives that we apparently began to take arranging pre-season games seriously.