I may not need it for the current attack, but it may be essential for my follow up attack.
Using your example, I may not need the BOM for 10 ARM attacking 1 INF. But on the following round when I send those tanks against the 12 INF in the NEXT territory, having the bomber changes my odds from about 50/50 for the battle, with me likely only having 1 ARM left after the battle to an 80% chance of victory and 3 ARM likely left after battle.
So, while I do not need the BOM THIS turn, I also do not want to risk it to SBR because I DO need it next turn. And by letting my BOM sit idle and guaranteeing that it is around for the battle the folowing turn, I will keep an extra 10 IPC of units alive and the end of my next turn on average, which is far better than even a maximum damage SBR. Plus I also guarantee that I won;t lose a 15 IPC Bomber.
18% chance of losing the BOM and then have a risky battle the next turn where you basically WILL lose all of your forces in exchange for 1-6 IPC of damage to your opponent, or have 3 ARM extra alive after your next turn AND still have your BOM.
So in reality, my request for a 4th option is actually far closer to “never SBR” than it is to “SBR when the BOM is not being used this turn.”
Doing an SBR JUST because the bomber is not being used this particular turn is in general a bad idea. Always have a purpose to your attacks. And that MAY include an SBR, especially on the turn before you are plannign a major assault to reduce the number of reinforcements that can be built… effectively when you can use the SBR as the openning salvo in your pending attack for the following turn.