The thing about guns is anyone of them can be spectacularly lethal in the right hands , like ships, if you play to their strengths they can achieve interesting results.
I'm not saying there shouldn't be anymore guns ever, but even something as humble and overlooked as the banshee can wreak havoc if you know how to use it.
Personally speaking I know I can take a pyra or junker with gat/ flak or gat/ mortar and rack up kills, but if you don't mix stuff up, it gets stale fast, so I will often take a load out that is more challenging to get kills with, and in the process perhaps stumble on a hidden gem (looking at you flare gun).
The other aspect is that some guns require a deeper understanding of their restrictions, like the flak or the lumberjack. Or a deeper understanding of the ammo types and how they affect the weapons, like lochnagar.