Does Walder Frey’s tendency to have lot of children – also inspiring his offsprings to do the same – give him an advantage in the feudal game? Is there any rational benefit in having so many spares? Or is it simply a symptom of his uncontrolled lustfulness and poor planning?

It gives him an advantage in the sense that he’s able to make a lot of feudal ties: Royces, Swanns, Crakehalls, Blackwoods, Rosbys, Farrings, Lyddens, Waynwoods, Vances, Hunters, Carons, Hardyngs, Lannisters, Darrys, Beesburys, Wyldes, Haighs, Blanetrees, Goodbrooks, Hawicks, Vyprens, Whents, Boltons, Leffords, Paeges, Braxes, Tullys – it’s a long damn list. So pretty much everywhere you go in Westeros, there’s going to be some family with links to the Freys.

On the other hand, that’s a LOT of money spent on doweries and dowers. And the Freys are only rarely marrying heirs or heiresses, so while Frey women would usually go to live with their husband’s families, there’s a lot of weasel boys who have no prospects of inheriting land who are living at home with wives and children who need to be supported.