depreciation is very much higher on a charter boat so it's a fair bet your boat will be worth about half its new price when released. that means you get back what you put in and the free sailing makes up for the loss of interest.
From the purely financial perspective it doesn't make sense.
Look into the finances carefully. In particular don't forget to include finance costs / loss of return and don't overestimate the resale value.
If you do the figures correctly you will see that it is not a stunning deal - and certainly not worthwhile unless you make extensive use of the 'free' sailing that is included. Extensive probably means 4 weeks or more a year.
I have met a couple of owners who were delighted with the scheme - but they used their entitlement to have several weeks sailing a year in exotic locations.
Clearly, these schemes are only offered to outsiders because the business model is somewhat flawed from the charter companies point of view. Otherwise, if it was such a great arrangement, the charter companies would keep all the action in-house and make the most amount of dosh they could for themselves - so they only allow outsiders to get involved to offload some of the risk and share in the costs.
Feels a bit like trying to gamble against the house when the odds have all been worked out long ago - can only work against you.
.. which is why the charter companies want to offload them. The only way it might be worthwhile is if you want to use a large proportion of your free weeks.
And have you seen the state of ex-charter yachts? Go down to Sunsail or one of the other charterers in the UK and ask to see any of the yachts they are selling!
Unless things have changed, the Port Solent boats are not a reasonable comparison. Up to a few years ago the UK was the last resting place for them, having spent years in the Med and Caribbean first. This is probably a wise policy on Sunsail's part as UK punters have a poor reputation in looking after them. Three and four year old Sunsail boats in Palma Mallorca, for example, are immaculate.