You technically never need to enter the Dark areas, but
enemies give more money, and completing these stages is the only way to get the best ending.
Not exact matches
The Rogue Mode is challenging but quite fun since you're
given a small budget to recruit your initial army, and then you have to win missions and kill
enemies to earn
more money, which you can then spend buy
more units between missions.
Still, it brought a few new and interesting things to the table, including Auto, to whom you could
give money dropped by
enemies in exchange for items, and Bass, one of the
more popular characters in the Mega Man universe.