If you charge exclusively on BP and put in 10kWh a month you will break even on the cost of the membership. If you charge from 20% to 80% once a month, you've saved yourself around 60p, which adds up each time you use it. So, if you do a lot of regular rapid charging, then it's worth it.
Edit: Just looked at the numbers;
One charge (20% to 80%) with £8 subscription = £34.52
One charge with free membership = £31.41
One charge as contactless guest = £32.23
Any further subscription charging within the month would be £26.52, saving £5 - £6 each time.
So I reckon if you charge from 20% to 80% three times a month it will be worth it.