Reuse of an address is the sole business of its owner. It's a matter of choice, for most people simplicity is the most important.
Pedantically, the reuse of address harms third parties... and the decision to send coins is the sole business of the sender. I don't think you can answer this questions with simplistic reductions to arguments about free choice: there are multiple people involved in the question, and their free choices may conflict.
I may sound paranoid, but the creation of a new address shouldn't never be done online and automatically. Creating a new private key every time you spend gives me chills.
New addresses can be generated without generating new private keys.
Monero, for example, has no address reuse at all in the blockchain-- it's required for the prevention of double spending there. It seems to do okay with it. The original bitcoin software never addresses w/ pay-to-ip; and even with addresses in use it the practice of reusing is somewhat inexplicable from a technology standpoint: it _really_ screws up your privacy along with that of people you transact with, and you can't reliably tell which of the payments you had outstanding were confirmed.... I think if it had been realized that people would behave the way they do, it likely would have been prohibited in the Bitcoin system from the start.
[I make these points as points of correctness, not to further argue it-- I think warning is more prudent, and will also have the effect of educating on this matter).