Resident Evil: The Mercenaries 3D isn't a game -- it's fan service. You see, there are two types of Resident Evil fans: casual ones that have fun playing a "survival horror" game and the hardcore buffs that really, really dig its absurd, kitschy, over-the-top action. I'm part of the latter group, meaning The Mercenaries 3D seems like a love letter to me.
However: If you're not part of that latter group, you'll quickly find yourself disinterested in what The Mercenaries has to offer. There's a reason why the original "The Mercenaries" game mode was tucked away as a bonus in the last two console RE offerings. Where are the puzzles? Scripted events? Where is the story? These are all features that have been severed, packaged into Resident Evil: Revelations (another 3DS game, due at a later date).
While some will rightfully take offense at Capcom's obvious "double dipping," it's hard to deny the sheer fun of knifing "zombies" in the knees and spin-kicking them to increase your score multiplier. The Mercenaries is meant for those gamers that find the core gameplay of Resident Evil fun and want to memorize maps in order to get the highest score. It's meant for the kind of player that thinks seeing HUNK fight RE5's Executioner while roaming a castle from RE4 justifies a purchase. By merging characters, enemies and levels from both RE4 and RE5, The Mercenaries 3D has effectively become the closest thing to a "Smash Bros." the Resident Evil universe has.