About 20 years ago at the school I taught in, Ripon Grammar School, in the MFL department we introduced a boy-girl seating pattern in Y7 and Y8. We did not extend it beyond Y8 because setting usually meant there was an imbalance of the sexes, so it was not possible for every table to have a boy and a girl.
At the time we justified it with two main reasons: the more important one was that would discourage chatting and silliness between boys, thus creating a more civilised atmosphere in the classroom; the second, less plausible, reason was to do with the idea that boys and girls may have different approaches to learning, boys being on average more competitive and greater risk-takers, girls being more conscientious and worried about getting things right. In MFL lessons, where pair work plays such an important role, who you work with is important.
Apart from that we did not have a seating plan policy in the department as such. I would let children sit where they wanted, occasionally moving distracted ones, whilst colleagues sometimes produced structured seating plans, especially with more challenging classes.
My department agreed that the boy-girl system was a sensible policy which encouraged good behaviour and I would commend it to other language departments.
Looking back on this now I think there is a third good reason for boy-girl seating. In school, if you observe children at break and lunch, the younger children especially tend to congregate with members of their own sex. To force children to sit with someone of the opposite gender in the classroom may go some small way to giving children greater confidence with members of the opposite sex. I would sometimes use the line "this is a classroom, not a social club" to justify not allowing friends of the same sex to sit together.
If you have reservations about boy-girl seating plans for any reason, you could try it out now and again. It's a good idea in any case to have children change partners from time to time.