Heart of Darkness in itself is rather mediocre expansion. It introduced a bunch of features to the main game, some of them were OK (like new colonization mechanic or naval system), some were broken (like crises or dynamic dominions), some were useless (like battle plans or newspapers). It could've been better with proper patch support, but then Paradox promptly abandoned the game in order to milk Crusader Kings II and Europa Universalis IV. The thing is that most players who are still playing this more than a decade old game is probably playing it with mods (because, lets be honest, the base game overall is very much diamond in the rough), and literally every mod in existence requires having Victoria II version 3.04 with both expansions. So this is not some optional feature pack that could be skipped, it's a MANDATORY purchase in order to experience this game in best possible way, and selling it separately IMO is inconvenient and somewhat misleading.
