I did this for the first child then didn't bother for any of the subsequent ones. It is expensive, and ultimately has been pointless so far - never used it at all for the one child who is registered. Their passports are just as good for showing they are British citizens.... YMMV
If you would like your child to have a middle name in the UK then this is an important step and money well spent, as trying to do it retroactively is MUCH harder.