We could.

You have to remember that some players have more that two weapon slots. Mouse wheel works fine with 2 or 3, but with 4 you start to fumble, since there's always a weapon you can't readily access. There's no option to bind weapon slot 1 to one key, weapon slot 2 to another, and so on, allowing decisive switching when you need to. I'd much rather use the 1-4 keys for this rather than vials, and rebind the vial keys elsewhere.

ROTFL, good luck using a mouse wheel to scroll in action through 3/4 weapons in a heated up fight. If you have nothing wise to say, don't say anything please.
About OP, i really want that feature, otherwise i won't never ever bother with a 4th weapon slot. 3 slots is already a PITA to manage without the hotkeys.

I used a mouse wheel with four weapons without issue, regardless of situation or tier. Some people just have an easier time with certain setups than others, and this is why there's a lot of ways to configure your controls.

For example: Some people believe that using a controller for Spiral Knights means you can't play it as well as you can compared to a keyboard, but in my case this game ''feels'' like a console game to me, thus using a controller actually made me play -better- because it felt more ''natural''.