Almost correct, Barry.
That's not the only reason but the main reason IMO
You're right, you could play any record either way. but the problem is the needle, back in the days needles were designed to be used in one way only. The were sort of blunt on one side so when you back-cue'd a record too fast a nasty noise was likely to develop. I understand that nowadays needles are not blunt on any side so shouldn't due any harm to your records. If unsure get a "DJ" needle or something similar from Ortofon or any other quality supplier.
I cue in every record as I want end of previous and start of next record to flow smoothly. Remember that outside the british Northern Scene DJ's hardly drivel between their discs
So next time anyone of you is dj'ing, balance the tone arm (about 2.5 on the scale when using a Technics 1210), get your own needles to make sure they're ok and if you cue in your record do it slooooowwwwly..