Thanks for your post! This is a great question and one of the most important things to learn for every user.
You're right: no address should not be reused in general. What you’re missing is the fact that this is only true for sending from it.
But any address can be used to receive funds as often as you like - until you spend from it.
As you can see, the current donation address from thetangle.org has received funds many times, but there’s no outgoing transaction ever: https://thetangle.org/address/UQTTFQTHRUXDUINIZXXXBMYCMJTMAKYJYYFSJZRYKXSNYZMITXVUCYAEDWPNHYSDSMEMZINLCDHRFWWNX
As soon as funds get sent from this address its security will be broken.
After that, it should never be used to receive funds again, because in order to get these newly received funds off the address, Mathieu would have to send from it again and that would be dangerous. You can think of it like a piggy bank.
You can put money into the piggy as often as you want, but once the piggy’s broken, you can’t use it anymore:
Now, if you want to set up a donation address yourself, I think it would be best to create a separate seed for this so you can avoid sending from your donation address by accident when you make any other transactions. You can then collect donations as long as you like.
But when you decide to take your donations off this address, you’ll have to put up a fresh donation address on your page.
You can’t prevent people from sending to your old address though, so you should publish a new address first and then wait for a little while until you expect no more donations to your old address before you move your funds from the old address.
Also, make sure that no incoming transactions are still pending before you send your funds from your old address. I think Trinity will protect you from this, but I'm not 100% sure. That might be something you could play around with if you like but only use very small amounts for testing stuff.
Let me know if you need a few iotas to play with and I'll send some your way.
Reusable addresses are something that’s being worked on by the IF, but coordicide is the priority for now and it will probably take some time before we'll see reusable addresses implemented.
But you can already read more about it here:
part 1: https://blog.iota.org/a-proposal-for-reusable-addresses-part1-bc6dbca84cbf
part 2: https://blog.iota.org/a-proposal-for-reusable-addresses-part-2-d83d328ff1b3
part 3: https://blog.iota.org/a-proposal-for-reusable-addresses-part-3-9ec6fa1929d7
I wouldn't be able to help anyone without thetangle.org-explorer. If you feel like you want to support its developer Mathieu Viossat in maintaining his service, please consider a donation to the address shown here: https://thetangle.org/about. Thank you!