OK, so I was thinking about this today and I couldn't make sense out of it -
Vettel has won the WDC. RBR has won the WCC.
Webber deserves a send off and Vettel could do with some rest considering the work he's got to put in next season. (said this in many interviews)
Vettel has been relentless and on the money for four years and he's got nothing left to achieve this year. Why not send Seb on a holiday and put Ricciardo in the car?
It would give Seb some well deserved time off, get Ricciardo a heads up on how the team works and get him used to some of the pressures of driving at the sharp end.
Also, it vacates the STR seat so give Danil Kvyat a go as well. If they had not signed him, they could have had a shoot out to see who can cut it.