There are very few instances where family boarding isn't allowed. I know people here say it happens all the time but in 'all the time' is a relative term. If it were me? I would buy it only for the trip home....and that would be just for the convenience of not having to check the family in at the 24 hr mark. Let SW do it at the 36 hr mark. If you get A spots, great, board then. If not, just board with the family groups.
Here's what I wish SW would do....those that are boarding in the family boarding group would have to move to the rear area of the plane for seats. Why? Simple. If you have a family group of, say, 6 boarding in that group, and they haven't paid for EBCI, it doesn't seem quite fair that they get their choice of seats when there are very probably a bunch of people boarding in the B group that did pay for EBCI. If you have 4 families boarding at this time, there could possibly be up to 30 seats that are taken before EBCI people can board.
Either that, or have families pay for the privilege of boarding inbetween A and B...maybe a per person charge, similar to EBCI.