I guess argument in this scenario is that the separatists could end up with a disproportional amount of influence on UK politics by being in the cabinet. The same amount of disproportionate representation that people were moaning about in 2010 when the Lib Dems got a sniff of power. Yes, OK, the lib dems have hardly altered the course of history with their interventions, but thats largely due to them ultimately acting in the interests of the UK whereas the SNP would be aiming to cause as much disharmony as possible.