I agree that the option to use parallel downloads should be configurable. Preferences/options are necessary to make any tool compatible with varying environments.
Settings can be changed via preflight/scripts/configprofiles, etc. for admins that have certain scenarios where parallel downloads should or should not be used. As an option, the onus is on the admin to use it at the appropriate time.
A quick glance at the proposed code changes seems like the number of concurrent downloads could be a variable as well.
Possible Preference keys:
ParallelDownloads -bool [Default: False] #Overall setting that auto-sets the next two
ParallelDownloadsManualCheck -bool [Default: False] #Control if manual MSC update checks should trigger parallel downloads
ParallelDownloadsLoginCheck -bool [Default: False] #Control if checks run when no user is logged in should use parallel downloads
ParallelDownloadsMaxThreads -int [Default: 5] #Parallel may be handy but not at the same threshold as other sites, make the number of threads a setting.