I agree that one of the things that 9 did best was to make all of the weapons actually useful. This has been a problem in pretty much every other MM game up to this point, where the weapons simply don't stack up to your pea shooter - this problem becomes twice as bad when you get the mega buster in 4 on and it just blows everything away (taking it out of 9 and 10 is an idea I back 100%). The weapons should be rewards and they should complement gameplay, being a good alternative to use to your default weapon in certain situations, and pretty much every weapon in 9 has its uses. Some of them are too good, even - jewel sattelite is a little ridiculous.
10 did another thing with weapons that I like though - make them powerful, but hard to use. Many of the bosses in 10, even if you use their weaknesses, still require finesse - managing to hit Blade Man while he's leaping from wall to wall with the Commando Bomb, setting Chill Spike traps for Nitro Man, trying to hit Pump Man in vain with the Thunder Wool because it's such a terrible weapon. Most of the weapons in 10 are still pretty good, my only complaint is that most of them eat up too much ammo, especially Commando Bomb and Rebound Striker.
10 has real-time weapon switching like in X, though, which no other classic game ever had, and that's a big deal for me. Being able to just switch to other weapons or Rush without going into the menu is really nice, even if you sometimes have to scroll through a few things to get what you want. It's still more convenient.