I should say that no stamp is worthless if you enjoy it. And even with selling or buying, it can be proved wrong. I'll just give you an example that is indirect but that I observed in the last period. On
ebay.de were some CTO lots from USSR and, even those have reached a value. For example a 3500 CTO stamps USSR all different 1960-1990 were sold for 85 EUR ( I know because at the beginning I even tough to buy them therefore I kept an eye on them). And there were several examples.
So, even the CTOs that the people are saying are worthless are sold (by even several bids... I think the one sold for 85 EUR has more than 12 bids). Of course that is 5 to 10% of what Michel says but... I think they were always sold around this value :)) And I'm not saying anything about the beauty of organize those 3500 stamps, read about their history, look at them and enjoy them.
So, to close the parenthesis, the Burundi are worthless only in the hands of the people that doesn't know to enjoy them.