My comments are from a UK perspective (and I feel that the UK is generally right when it comes to LVOs).
Unless there is some good reason to believe that the met data you were getting is inaccurate then you 'believe' what they tell you and act accordingly. If the RVR is u/s, pass the reported met vis to the aircraft.
What you can see with the eyeball is an accepted way of making weather observations (although in Europe it is becomming more common for automatic equipment to produce the data and both met observers and ATC sometimes seem to forget how to work without AUTO gear).
Don't know what the rules are in SA but in the UK we don't 'close down' arrivals in poor wx - we pass the met info and ask intentions, then try to accommodate the crew's wishes.
In the UK the rules are strict about RVR - this follows incidents in which the IRVR was though to be inaccurate (reading too low) because both ATC and pilots could see for miles but the slant visibility on approach was poor (and being accurately reported). This was in patchy shallow fog - don't know whether this is common in SA but it sure is in the UK in spring, winter and autumn!
In UK, rules are that RVR is given soon after contact with approach and then any changes are passed - this is often done by broadcasts and specific calls to aircraft about to start an approach. Sometimes a crew will ask to be notified only if RVR falls below a particular value - according to the book ATC should keep giving updates, but often don't to avoid adding to crew workload.