I personally would not consider them to be more valuable than a comparably sized boat (especially when by size I mean more than simple length on deck), but I would guess that the price for these boats somewhat reflects their argueably undeserved cult status. I suppose that you could argue that their price may reflect a number of possible factors as well, such as:
-They offer a lot of room for their advertized length on deck,
-The offer a lot of displacement for their length on deck,
-New they were on the more expensive side of things than most boats of the same length, build quality, or displacement.
-They have picked up some of the perception of being well constructed that, argueably, derives from the later model Island Packets and more to the point, the marketing that went with the later model Island Packets.
-Asking price is not sales price and so the asking prices may be inflated by one owner seeing another owner''s asking price. (Quite a few years ago a broker who at the time specialized in selling Island Packets said that asking prices generally exceeded their actual sales prices by a larger amount than he saw with most other boats that he sold),and lastly
-There is no accounting for taste (he said with tongue firmly in cheek fully expecting to be blasted by someone who finds these boats right in line with their taste in boats).
Their lack of speed comes from a lot of factors. The basic design of an Island Packet crams a huge amount of weight on a comparatively short sailing length. This may result in a bit more comfortable accomodations for a given sailing length but at the price of motion comfort and performance. An abundance of weight for a given sailing length makes for a comparatively deep canoe body and blunt ends, which combine to produce in a whole lot more drag than would occur in a boat with a longer sailing length for the same displacement (especially in a chop or going up wind). The deep canoe body means comparatively short spans for their keel foils and with their choice of a long keel the results are a huge amount of drag due to large amounts of wetted surface and a massive tip vortex for the comparatively small amount of lift generated.
Offsetting this high drag and weight is a comparatively small and inefficient sail plan. It is a sail plan that works well reaching in winds in the mid-teens and perhaps slightly higher. In higher winds, the comparatively high drag and inefficiency of the sail plan means that a lot more sail needs to be carried to maintain speed than the stability of the boat can resist comfortably. In light air the high drag an small sail plan is a killer.
Adding to the basic speed inhibiting design charateristics, the layout and choice of deck hardware on the Island is poorly designed to allow performance optimization. Many IP owners seem to opt for sails and sail handling options that are optimized for conditions other than they typically sail in and so further inhibit performance.
Collectively this produces boats with a genuine limit on their preformance. This seems to be acceptable to the people who buy these boats for whom performance does not seem to be a high priority.