Some time in the 1970s, I remember it being used to platform a Holyhead-bound local DMU while a late-running loco-hauled train (probably from Euston) used platform 3 on the other side of the island. It had no number then; I think it was announced as 'the other side of platform 3'. I'm not sure if this was a regular thing - I only saw it happen that one time myself. Odd that it wasn't given a number, as it was evidently a usable platform, even if would only be used when trains were running out of course.
I also remember the centre roads being used to stable terminating trains. I think there were a few Eustons that terminated at Bangor rather than Holyhead up to about the 1980s.