Didn't mean to be patronising Tyro, and certainly not with you.
I just did too fast a reading and read some stuff explaining why engines run with batteries out. I thought this was not necessary here. Hence my coment.
At least, I did not include a smiley bumping its head against the bricks...
I guess it made me remember a student pilot of mine, who in his second solo had an alternator failure. he landed normally and was excited because he was scared of the engine flaming out. I gave him homework inmediately, and asked him how on earth had he passed the ground school exams.
Of course, going to CONF 3 before batteries deplete is a good thing to do, not covered in any FCOM by the way. And it is very unlikely that increased fuel consumption will make it non advisable. Even if you are in the most critical point of an ETOPS flight, you can wait 25 minutes to lower the flaps while descending to 20,000 towards the nearest suitable alternate. The reserves would suffice (fuel burnt approximately twice)? I guess it would.