The weapons are intended to be unequal, and it's the player's job to use the proper weapon for the situation.
Automatic is amazing when you have the rings to burn. After a 1-up or any other time you have more than 100 rings, give auto a shot. You will be surprised how good it still is. Almost all the stages have areas with large amounts of rings to allow you to try your luck with automatic. As long as you've got ammo, it's still a terrifyingly strong weapon.
Bounce is amazing in small areas or area denial. It's also quite useful when you're in a small passage running from someone and you want to hit them without looking back. Throw some bounce rings at the far wall and they'll come back at him. I still find it most useful in the flag room in Silver Cascade, and other similar rooms with low amounts of space. You can turn such rooms into living hell.
Explosion is a great all-around weapon. Used at medium range, it packs a huge punch and can launch your opponent quite significantly. Used at point-blank range this is basically undodgable. If anything is possibly overpowered in 2.0 match, it's probably this.
Grenade is good for area denial and well as covering yourself while running. Most players won't watch you drop mines as you run, and will get hit, allowing you to escape clear, and with an extra 50 points. It is, as previously mentioned, mainly designed to allow you to defend your base in CTF, and it's quite good at that purpose if you know what you're doing.
Rail is still great for long-distance sniping, and is basically the only long-distance weapon in SRB2. It's also got more of a punch when it lands, which can be amusing in levels like Nimbus Ruins for launching other players into pits and other hazards. However, Rail now has to contend with the other weapons, which makes you quite vulnerable if someone comes up close to you and you miss them. If you miss and they have Scatter, you're likely in for a world of hurt.
Scatter is designed to be the point-blank weapon, although it's also pretty decent at medium range. The main goal when designing Scatter was to make a Shotgun-style weapon where aim wasn't as important and the important thing was making sure your opponent is somewhere near the center of the screen. Again, like Explosion, Scatter is a good all-around weapon for most situations.
While it's easy enough to just always select Explosion or Scatter and use them always, you're gimping yourself if you don't select the other weapons when the situation calls for it. All the weapons have a use, and if you use the best weapon for the situation you WILL do better in the new match system.