Pokemon-go – What are the spawn rates for any given spawn point

How many unique Pokemon are there?

Please see the most up-to-date and continually updated answer to If I start now, can I catch every Pokémon in Pokémon Go?

If there is less than 250, is there any point to having more than 1 of any given Pokemon?

Candy

In order to power up your Pokemon and evolve, you need candy. Candy is one of two main resources involved with powering up, evolving, and gaining a secondary charge move.

Catching provides candy based on the evolution stage:

• 3 candy for base stage
• 5 candy for second stage
• 10 candy for third stage

Raiding

Since the release of Pokemon Go, raiding has become one of the major aspect of the games. While batting the raid boss, having multiple of the same Pokemon can provide a boost when it comes to exploiting type advantage (e.g. 6 Aura-Sphere Lucarios vs Tyranitar)

PvP

Pokemon Go has PvP battle leagues, all of which have different thresholds of allowed CP. There are many top-meta Pokemon, such as Registeel and Swampert, which perform well in various leagues. In PvP, one benefit to having multiple of the same Pokemon is to have a variety of CP ranges to have one in each league.

Borrowing from @Codestrife's answer here

The rustling leaves usually indicate that there are pokemon near that area. Going and walking around near that area gives you a higher chance to encounter Pokémon.

Keep an eye out for rustling leaves, hinting at where critters may be lurking and simply hang out in a suspected region for the increased chance of an encounter. - Source

Therefore, the reason you don't always see Pokémon in rustling grass is because there is only a higher chance to encounter them there and not a guaranteed chance of encountering them. You just have bad luck when you don't encounter them.