Let's see, what can premium players do that free players can't:
1. Fly - IIRC free accounts can use teleport, so they can still travel. It just means you have to find a psychic pokémon to travel with.
2. Visit Saffron - a city which isn't even finished yet, it offers 20% lower prices on all items (roughly) and has a shop where they can buy evolution stones.
3. Feel good about helping keep this server up and running.
4. Most Premium players tend to be players that keep playing and help new players, so they're also rewarded for their loyalty in ways I don't have listed.
5. Own houses, which the furniture store isn't ready yet for (next update fixes that).
6. Some hunting locations, NOT MANY, that free accounts can't get to right now.
So you think that Premium players get a massive advantage over free account players? You need to stop and actually play the server and see that it's not like that.
Also, Diamonds can be used in the game shop, by saying !shop, which has some items they can buy that aren't overpowering, but will help them.
The donation/shop system is one of the things that I feel is better than all other pokémon servers, because it's fair. Not overpowering. Fair.