Do you mean part pay with avios on a paid ticket? If yes, then you didn't pay for the avios. BA paid you to buy back your avios.
Correct, so this is poor use of Avios, since you are getting some 0.73p value per Avios, and in my opinion you should be looking for 2p value or more.
However some part pay options give over 1p: sometimes BA has a deal on in this area, about twice a year (not at the moment) where the effective value is over 1p. Also with some part pay combinations, notably with Iberia and Level, that figure is much better, so you can save (e.g.) 25,000 Avios for an extra £150, a rate of 1.67 pence. The BA equivalent would cost around £300. Finally some BA part pay rates are neutral, so near the 1p mark, in which case it may be worth paying more cash tactically, if you can get more 2p value elsewhere or can combine with a 2-4-1. Plus of course if the alternative was cash (due to having insufficient Avios for highest Avios rates) then you have to be wary of comparing a relatively flexible Avios ticket with an inflexible cash ticket.
[Though be careful, Iberia redemptions cannot be cancelled away from Iberia,Level,BA, Vueling, Nostrum, plus multi-partner bookings]