I can tell you that at least in the games we’ve played it has made a huge difference at crucial moments, playing a big part in helping to turn the tide of the war. For the Axis, being able to do an SBR on the UK or India is almost necessary to facilitate their capture - more IPCs spent on repairs to ICs means less infantry available for home defense. Same idea applies to a Russian invasion, especially once you get close to Moscow. On the other side, it can cause Germany to have fewer units at hand during an Operation Barbarossa, and it’s useful for the U.S. to use on Japan as well. To those powers starting off with relatively weak economies, such as Italy and ANZAC, being victim to a successful SBR is often the second to last nail in the coffin for them.
Notice that I’ve just been referring to SBRs on ICs. To be honest with you, in my personal experience I haven’t had many instances in which I would rather dedicate one of my stat or tact bombers to a SBR on a sea or air base instead of using it in general combat. At most, you can do 6 points of damage to a base, and since they are considered operational at up to 2 points of damage, you can only hope to make your opponent pay 4 IPC per turn to get them back up and running.
So, at least in my mind, SBRs are definitely worth it if you are using them on ICs, and are most effective on major ICs. However, when considering using them to hit bases it’s probably more worth your while to commit the rest of your strat and tact bombers to general combat. That’s my 2 cents.