Frankly I don't think any station on that line *needs* 8tph off-peak, but Plumstead is essentially in the same category as Charlton. It saw its usage increase by 41% last year, and I think that is probably still an under-report due to many passengers buying season tickets to Abbey Wood (same price) and sadly ticketless travel to local stations. Plumstead's service is generally good. It has 8tph off peak (as you say) but in the high peaks the service can be worse (e.g. a 20 minute gap between 1820 and 1840 from London Bridge).