You may have your own reasons for travelling via this route
Agree re. not having to drag luggage / family across London on the Underground.
Mrs Bungle and I travelled on this route from Southampton not long ago. It was worth every penny of the extra fare to NOT go via London and be able to sit in the same seat all the way from the south coast to Yorkshire.
A word of caution, however: Our 0946 departure, on a Saturday, was rammed full of cruise passengers (of which we were two); the Voyagers are barely capable of taking the massive amounts of luggage these people bring, and the train was already very busy leaving Southampton. Anyone travelling around this time and not in First would be well advised to try and manouevre their way to be one of the first to board, and try to get the coach (D?) with the extra luggage storage; it was pretty chaotic elsewhere, especially with the number of people who evidently travel by train once in a blue moon.
We heard the TM say to another passenger that it is a common problem and that the cruise companies themselves tell the TOCs when to expect high passenger numbers, but little is ever done.