I did it, but it never made a noticeable difference to my insurance (in the region of £10-20 over a year, for a premium of ~£1200).
As mentioned by DampDog above, if you've had a decent instructor you will have most likely covered the majority of content on there anyway, and if you've been on any decent sized dual carriageway, then motorway driving is very similar.
Not worth it, it's just common sense and basic driving skills.