It's not too bad on certain routes
There are indeed some routes that doesn't make any sense on avios but there are some other cases too. What about flying to Mauritius? You could pay about 350€+ or you just spend a few points and that little bit of taxes.
JNB-MRU one way would be 32€ + 8500 Avios.
JNB-CPT one way would be 32€ + 6500 Avios.
If I was based in JNB or got a cheap flight to there and wanted to go on to MRU it would make indeed a lot of sense to use avios. It's always case dependent.