I don't have the figures, but I would be surprised if it is "very few". I have exactly the same cabin baggage allowance in CW as someone on Basic WT, and with a few exceptions, including one recent silly experiment, I've done a whole travelling career on HBO. Indeed I usually take one bag, not two. And from what I can make out quite a few other FTers rarely or never check in bags. Now I accept for the occasional traveller, perhaps on a rare family trip, HBO would be a rarity, but what we do know is that 25% of BA's passengers have not flown with BA in the last 2 years, whereas 39% of BA's passengers have flown 5 or more times in the last 2 years - that group could probably adapt to these fares quite easily on shorter trips.