Lighter armor and the inherent fragility, but high-speed/mobility and low fire-rate with high-velocity projectiles, for me.
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This is an example of a specific set of preferences. It is how this player wants to experience the game. Now if a heavy armored, slow moving, rapid fire weapon build was inherently more effective it isn't necessarily going to make a person want to play that entirely different way right? That person is just as likely to simply not want to play the game at all. This is a situation where a game has to be balanced well enough that using a build of each type is a viable option. Sure, there may be better and worse versions of that low fire-rate with high-velocity projectile weapon, but if that whole weapon category is an objectively weaker choice it dissuades a player from using an entire play-style which is can be a part of alienating an entire type of player.
Balancing such things is not easy, and it will never be perfect balance. It just has to be close enough. When it isn't the developers must re-tune until it is. It's an ongoing pursuit.