Live in Pullman, WA? You're flying Alaska anyway. Salt Lake City, UT? Get yourself a Delta number.
The tricky places are where there are lots of competing carriers. San Francisco with it's...pseudo-hubness for a number of airlines, Washington DC, etc.
That said, when I need to get a flight somewhere else and there's no BA or AA flight convenient I just get the flight I want to get and don't sweat the loss of status building.
As a SFO-based frequent flyer, if you fly to numerous destination (instead of doing the same commute each trip), your choices are likely: fly United or take an extra connection.
Unlike say, Denver or Salt Lake for United and Delta respectively.
I'd also put SEA in this category, especially with Alaska and Delta no longer playing nice in the sandbox.
Maybe one day I'll book a flight with Avios, and when I do I'll have enough for sure, but I don't treat them like a perk.
Agree with the BA tools/site being terrible, it seems to be pretty consistent that this is the case across airlines.
Another thing though about Avios is BA's constant errorsion of benefits. Every year or so they send out an email saying 'we asked our customers and they said they wanted more choice', followed by saying they're removing free drinks and meals, reducing the hours hot food is available in lounges etc. I'd respect these decisions much more if they were honest and said 'our shareholders want us to cut costs!
Basically, I add my frequent flier number to the booking form online and that's it. When I hit Bronze (it's a couple of long flights or so) I don't need to do anything, I just get to skip the queue at checkin and (airport depending) security.
There's no sales, no emails, no upsell, my (travelling) life is just better after a couple of flights.