When loading the BiocInstaller package or packages which import BiocInstaller (e.g. affy) without internet connection, this results in a long timeout (depends on connection settings) and warns:

Warning message:
In read.dcf(con) : unable to resolve 'bioconductor.org'

The problem is, that packages importing BiocInstaller and which should be usable in offline modus suffer from the same issue. The timeout causes further problems when combined with time-critical operations and internet connection is not an option for some scenarios (e.g. sandbox or test environments).

From my point of view, internet connection should be tested only when running corresponding functions, not when loading the package or packages containing some functions requiring internet connection.

I agree and will make that change. If you're using the release version, an undocumented feature is that you can set a system environment variable or R options(BIOCINSTALLER_ONLINE_DCF=FALSE) e.g., in ~/.Rprofile.