I'd only ever purchase Avios if I was certain I would make a significant saving on the purchase price versus the cash price. For example if, theoretically, I didn't have enough Avios to use a 241 to The Maldives but I had a GGL Joker and I'd checked that the seats could be made available then I might jump in and purchase at that point.

I have to say though, I'm increasingly finding that cash tickets are better value than Avios redemptions in almost all circumstances.