That might be the case in some business models, but in a truly agile environment delivering customer facing systems, test environments must reflect the live setup, and not just be dumped on retired tin. Having your customer facing systems (i.e. transactional website) on a cloud environment allows for seasonal scaling (managed by a team to avoid the holiday issue mentioned!), and temporary provision of "like-live environments" for performance and volume testing (which should be carried of on a very regular basis).
Whilst Trevor's thoughts may be applicable for many backend systems (finance, etc.) which do just plod along, many industries require burstable systems. This bursting can easily be scripted and triggered using monitoring tools. Whilst I agree, this is not a panacea, it isn't the purely political solution implied either.
The right tool for the right job!