Re: Should websites refuse to send coins to an already used address?
reuse also harms _other_ users in Bitcoin, who do care about privacy, and this degrades the fungibility of coins; since coins which are linked to this or that party are clearly different, and differently valuable than coins which are linked to other parties or are not linked.
If that's so harmful for Bitcoin why there's was never prohibited in Core?, I mean, the address reuse should be disabled if you are 100% right. 
Btw, how do we can have working donation addresses this way? Only using it for sending the inputs to another random addresses?
"Mined addresses" (vanitygen) are harmful then too?
This is very shocking for me.