I guess it all depends on what you're looking to do for your game.
Many things are possible with the official version, but in many cases Ply's Mod can make things many times easier on you. Take a Cash System for example. Conjured Entertainment proved that it could be done, but the same system was achieved using Ply's Mod in half the code and with twice the power.
Ply's Mod can also make custom GUI menus (these are a little harder, but they're very possible) for whatever purpose you see fit. Instead of requiring the player to press a certain key to purchase a weapon, why not make a shop GUI where they select the weapon they want to buy? In all honesty pressing a key somewhere on the keyboard to buy the weapon I want is about as awkward as pressing Enter to open a door. My hands are already on WASD and the mouse... if I'm running away from something, I don't want to have to let go of one of those just to reach over and hit Enter to open a door and shut it behind me.
Or maybe you're tired of having room for only 9 weapons... I mean, saving the universe is a lot of hard work and you need all the fire power you can get your hands on!! I've made an inventory GUI using Ply's Mod that allowed for MUCH more weapons than that, and you were allowed to bind any of those weapons to 1,2,3, and 4 for easier access.
IMHO, Dynamic gameplay brings a lot to the table. Being able to change the game any way you fit, whenever you see fit.
Ply's Mod concentrates on bringing gameplay in FPSC to another level. If that's what you like, then the least you can do is try it out and see if you like it. If not, the official version isn't exactly going away anytime soon.
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