It isn't overtaken at Eastleigh in the down direction, so even for passengers from intermediate stations from Waterloo it provides a semi-fast service as far as Southampton, which it did before they linked it to become a through service beyond there.
There's a logic to what they did though. Prior to the link up, there was a separate Brockenhurst to Wareham service, this provided the all stations service between those points. There was an hourly Waterloo - Weymouth service, and an hourly Waterloo - Poole service.
The Brockenhurst to Wareham service ran in timings suitable for passengers to change out of a fast service at Brockenhurst for the minor stops beyond there. The Poole service provided the calls between Southampton and Brockenhurst.
Fast forward to the decision to run two tph to Weymouth. A possible solution was to just extend the Poole service, with all its existing calls, through to Weymouth, and perhaps run a shortened Brockenhurst to Poole stopper, but they wanted to speed up the second Weymouth train, so they removed the calls at stations between Southampton and Brockenhurst, and gave them to the new Poole service which extended from Southampton.
So what this did was link two separate services (while leaving them in their original timings) by adding a new short section in the middle, so what you have now is a service that is caught up at Southampton and overtaken, but allows for passengers to change from the fast service for places such as Totton and Ashurst, and it also allows for passengers in the down direction from Shawford and Eastleigh to change into a fast service to Bournemouth and Weymouth.
That's the down direction explanation, in the up direction it is far simpler, with only one 'overtake' at Eastleigh.
Some people think that the Poole service should only be advertised as far as Southampton on leaving Waterloo, and as a Brockenhurst service once well into its journey, in a similar away to the up direction train being advertised as a Farnborough service west of Southampton.
Breaking the route at Eastleigh? Well that would remove the main Eastleigh to Southampton service, absolutely no point in that at all. I think when you suggest changes you really must consider ALL the intermediate flows in both directions...