I disagree. Yes plugin authors SHOULD do that, but WordPress plugins should be able to declare a parent plugin that must be active. That way, when BuddyPress is deactivated, my Welcome Pack plugin is deactivated automatically. It also protects against plugin authors who do not know how to do this properly; either through lack of PHP experience or ignorance. There’s a whole range of WordPress plugins out there, and some are really well written, and some are really poorly written.