Could you provide a concrete example to explain why reusing addresses by A will affect B if B always carefully choosing address. and how both A and B never reusing addresses prevent it? I'm still not so clear about it.
Since the drawbacks are very apparent, IMHO you need a very clear explanation about the benefit and why the benefit is far more important than the drawbacks.
http://blockexplorer.com/address/1Lukejrwhew7sj4TvWCKksaVo7aLpedHDtFollow the coins back ~12 hops to where they were generated, then follow forward where they were sent to "A". Easy to identify the recipient and owner. Backwards, not so much.
Now if B's next payment with the change from that transaction is to "free Tibet", buy "recreational substances", or pay a hitman to whack a business partner, association with the transaction A may reveal identity. When A is shared and reused, as in "this is the donation address for Eligius", any separate-channel information about someone making a donation to Eligius can be used with this known address to reveal a path to their money.
How do you know it's a change rather than another transaction to others? Why sending BTC to a donation address will disclose my identity and address? People all around the world send to well known address of 'just-dice' and never worry about their identity is disclosed.