The problem with mechanics like that is that there is always an "optimal" choice, and everyone will figure out what it is and always use it. This happens in most games where you can determine your attack choices before combat. This leads to stale games where only the best mechanics get used and all the others just end up lie there forgotten. For instance, in the old system, Automatic-Infinity and Rail-Explosion were basically all that got used, despite the massive variety the system really offered, because the other combinations were either overpowered or extremely weak.
The advantage with a traditional FPS system like we're using now is that you're going to end up using what you have, not what is necessarily optimal. Sure, Bounce may not be the best weapon in the game, but if you don't have any other weapons available to you you're going to use what you've got. Even better, because of the ammunition limitations, if you keep using your good weapons too much you'll have to resort to backups. This provides gameplay variety because you can't just use the optimal choice all the time. Even better, the different weapons are optimal in different situations, encouraging the usage of all the game mechanics. This helps prevent the game from getting stale because it adds variety to the encounters.
Sure, it's not as creative as the weapon stacking system was, but you actually have more weapon variety in this because all of the weapons get used at once, instead of 1 or 2.