I always thought that most of what Merlin were doing around the time Spinball was introduced to the park felt very 'temporary'. Those rides weren't very well integrated. They'd put the 'secret weapon' series on hold (likely because Wardley was sort of in retirement at that time) and each new attraction they introduced seemed to be quite standard and unimaginative. Given the financial climate of the UK at that time, I guess all this sort of made sense from a business risk POV.
It'd be relatively easy to dismantle, would leave no detriment to the landscape, and it's exciting enough of a ride that they'd be able to sell it on to a younger second-class park.
I've never really considered Adventure Land a proper area of the park in the same way as some of the others, as there's no real theme or design to it. To me, that plot of land doesn't seem the right place for a coaster. I'm not necessarily saying the coasters are eye-sores to the landscape (let's not even go there lol), but I just feel that a small show-building of sorts would blend in with the surroundings better if integrated in with vegetation and foliage etc (in a similar way to how Duel is; if you've only walked past that facade and never been in there before, you'd have no clue how HUGE that building is).
Since it's near the entrance to the park, and has a great view of the towers over the lake, I'd probably give the attraction a 'legend' sort of storyline.