Just because Brisbane rate him highly, doesn't mean they won't use their position of power to get the best deal for the club, especially if he decides Brisbane. They will know Tippett's leaving regardless, therefore Adelaide would ideally want a deal to be done instead of leaving him to go PSD. Are Adelaide going to get a better deal than pick 8? It's what Brisbane was prepared to offer him this year, and with his concussions, is his value really greater than it was last year? Brisbane got pick 12 for Clark, and he kicked a similar amount of goals in 2011 to Tippett this year, playing a similar number of games. We said no to two of Polec/Redden/pick 8 last year, and I don't see how that's changed this year, except Tippett is now out of contract and has concussion issues.
EDIT: FWIW I still think Tippett will end up at GC, and this is all his manager's work to get GC's offer up. GC know he wants to go there, so are likely offering him the worse deal of all teams,