Benefits of having an army of Raticates?

While I wouldn't bother using stardust to level up your Raticates, I would definitely use up your candy evolving them. They give 500 experience each time you evolve a Pokémon (1000 when using a lucky egg). Therefore, while they may not be the best Pokémon out there, they give you a nice influx of experience.

TLDR: Evolve Raticates for experience, save your stardust for your best Pokémon, and turn the rats into candy when you get too many of them and need room for other Pokémon.

Note: like @wax eagle said, saving candy and rats to evolve at once with a lucky egg is a good strategy to level faster.

As to your first question, you are correct, it is probably a bad idea to use your stardust on weak pokemon as you will catch other Pokemon with a far greater potential than raticates.

Also the fact that you are a low level trainer (assuming this based on your highest raticate being 325cp) means you should hold onto your stardust. This is because as your level increases you will catch higher and higher CP ratatas which can be evolved using only the candies which you have plenty of.

However if you are planning on controlling many gyms right now, then it may be best to evolve as many as you need and power them up as much as you need.

As to your second question, you should evolve your ratatas when you catch a high CP one, then you evolve that and it will be stronger than the ones you have now, since your trainer level is increasing. You can transfer your lower level raticates as you get better ones for an extra candy.

Dude, if it floats your boat, spend stardust on what you want. There's no specific way to play the game; if your goal is to have an unstoppable army of Raticates, which sounds awesome, then by all means DO it! Or if you're like my brother and want two hundred and fifty Drowsees all named Pound, make it your quest.