Agile does not mean paper napkin architecture. On a system that is large scale the team needs to be thinking about architecture at all times. This does not necessarily translate to lots of upfront architecture.
But key architectural / design issues should be verified using spikes or conceptually to some extent, to to address key tenets of architecture like layering, tight coupling, scalability, testablity etc.
A paper napkin architecture is just a way the pair discusses about the story before they start developing the story.
Agile teams esp XP teams consisting of senior developers can pull off with Just in time architecture. But in most cases the team consists of a mix of developers and to expect every member to contribute to the evolution of architecture is a challenging task.
An architecture vision and some detailed archicture analysis ( may be for a sprint or two) is good for the overall health of the system.