Not that rare in 1978 - i was on Buenos Aires Star during 1978, which was on the Southampton to River Plate run along with Brasilia Star and Montevideo Star, then Gladstone Star - before they switched to Newhaven. At the same time the Deseado and several Lamport & Holt ships were also on ECSA run although not from Southampton.
However by early-mid 1980's it was becoming rare to see such vessels in Southampton.
I worked in the port area from 1969 to the end of 1980, and by the time CoA arrived for it's brief visit, it was getting rare to see this type of vessel in port. I certainly never saw another Ellermans classic again.
The ships you mention were the last of the line and sure were calling at Soton, but maybe one or two a month, not one or two a day like 10 years earlier.
You were lucky to have worked on such ships, though I doubt you thought so at the time.