You can't police server quality, so unfortunately there's always the problem that a server you donate to starts becoming crap, or shuts down or no longer does any of the things you donated to it for.
But that's a risk you take.
Personally, I find donations to servers to be perfectly fine. If the server is a lot of fun, is well maintained, and is often (if not constantly) hosted, then there's no problem with donating.
Donor rewards are fine too, in my opinion, so long as they aren't "corrupt".
That means that you don't give a donor Admin priviledges, or let them get away with breaking rules or whatnot, simply because they donated.
Things like aesthetic upgrades are better. Imagine some secret Speedkarts or something that only Donors can get in a Speedkart map (Give Donors a Key-Item and set a new or certain speedkart to only spawn when selected with a key). Give someone a special symbol next to their name, on those servers where people use those Silver/Gold admin symbols. Give them access to a unique (but not unfair/unbalanced) weapon or item in a T/DM.
I don't see the point of donating on Blockland servers, it's what $6 a month? You can easily pull that out of your pocket by a month. Just save $2 a week or for 3 weeks or something and bam. The only game id see fit to donate would be monecraft servers since those servers are actually expensive like $15-$20 a month.
Some people can use the help in providing for their server host. Donations allow that.
And even if they don't need the donations to cover the costs of hosting (They might have money enough for it, or host for themselves), there's no problem with it.
Donations can be solely about showing support for someone.
If someone runs a top-quality server, maybe I want to donate a couple pounds to show my appreciation.
And if that also motivates them to work harder on the server, or to use that money to hire collaborators for producing Add-Ons for the server (Think back to custom speedkarts or weapons/items), then all the better. And if they do nothing with my money, or shut down, then I run that risk. It's a donation, not a purchase.
Really though, there's no problem if someone makes a profit out of hosting a Blockland Server.
If they're doing it legally, then there's nothing wrong.
If other people are happy to spend their money how they want, then let them.