The authority where I work only validate using the Capita provided database at present, which is not updated as regularily as I would like. I've also had a call logged with Capita for almost two years now, due to Validated addresses putting some data into incorrect fields, for example Flat Number into House Number, where House Number does not, and should not exist according to the BS7666 rules about PAON's, and SAON's.
As mentioned, having correct validated addresses in the system means that the sibling links can be kept, and also the data is cleaner to work with for both school and LA.