I think the value of the buff hammer over an extra ammo type or two has been explained multiple times.
No the point is I can only speak for me and not you. I don't value a single, static unchanging style of play.
I've seen 3 crew fads in my time in this game, and when the magic element in the 3 engie loadout gets nerfed, and it will, someone will sit down with a damage spreadsheet and work out another.
I value it even less when you have low level players latching on to this because "that's how you win", who don't realise it's simply gaming the game because they have no idea about what the game is about - the gunner has 3 slots by design - and come nerf time they'll be left groping about.
The latest nerf to the mercury is in part caused by the 3 engie buff crews in competitive matches, yet the fallout spills into general play, so I value the buff strategy even less.