Most of the doubling up on Bournemouth services over the last year or two occurred when trains were only running 2 hourly. In normal pre-2000 timetable there was only ever a very few doubled sets, one each way maybe, which didn’t run every weekday either.
Even when XC were running hourly, there were doubled up sets. I was certainly impressed for a while, turning up to be continually greeted with a 8/9/10 car XC service at the platform.
However, with the current state of XC rolling stock deployment, I wouldn't be prepared to use them all the way through to Manchester Piccadilly. I've used them as far as Coventry before, that was bad enough. In particular on the return journey that also had football traffic, resulting in First Class being declassified due to the on-board crush - all seats full and noses pressed to the wall in the vestibule areas.
Quite simply a four carriage train is ridiculous from Bournemouth all the way through to Manchester. As others have stated, the amount of standard class seating is barely more than a local train. In fact, some local services probably now have more seating capacity than XC does.
In 'the old days' of the 1990's, I can certainly remember boarding an Intercity 125 at Reading - destined for Bournemouth - with eight carriages, including a buffet car. Nowadays this is usually four carriages with no full service buffet car. What a huge regression when passenger numbers are higher than what they were back in the 90's.
If some people know a service is likely to be more than full and standing, they'll likely avoid using that service again, which in the long term doesn't help grow passenger numbers.
I also suspect many of the on-board XC staff must get endless complaints about the lack of capacity, for which they can do nothing about. Which is not fair on them, to be honest.