You don't have to fly BA (apart from any 4 sectors on BA metal), or use LHR for the most part, to enjoy OW Emerald or the benefits of the BA Exec Club program @:-) I only flew the 4 BA sectors (on a planned trip to LHR) whilst all of my status flying & Avios earning has been exclusively on CX/MH/JL. Very happy with this solution
If you see upgrades as part of the value of the program, BA EC is terrible. First, you have to fly BA to upgrade, second, you have to pay taxes on upgrade and third, you need to pay for WTP to get CW (pay more, get less).
Since I mainly use miles for upgrades instead of redemptions, it makes no sense for me. On the other hand, if you use miles for redemptions, BA might be for you. From what I understand, even partner redemptions (PITA on QR) are relatively easy with BA EC.