I'm probably not typical of posters to this forum since I'm not a frequent flyer (BA Blue at the moment). Years ago, I used to fly on whichever airline offered the cheapest and/or most convenient flight. I found that, as an infrequent flyer, I never collected many points in any single airline loyalty scheme. Often, the points would expire before I could spend them. I therefore decided to focus on BA since I'd had very good service from BA in the past (I started flying BA in the 1980s). That hasn't stopped me from flying other airlines when BA don't offer that route (for example, EasyJet for BRS to EDI and BRS to TLS, Eurowings for LHR to CGN).
My first trip since the COVID-19 restrictions were lifted was to FRA, I was very tempted to book Lufthansa, but ended up booking BA because I could use my Avios to upgrade both flights to CE. My next trip is going to be to WAS. I was very tempted to book Aer Lingus via DUB so I could pre-clear US immigration in DUB but ended up booking BA again because I was able to book a flight and hotel for less than the cost of booking the hotel direct, plus I was able to upgrade my inbound flight to CW with Avios.