My stand is clear. Frontend credits should be optional regardless of how they are written HTML comment or a link.
Both theme and plugin credits should be optional.
There is no, to me, good reason why themes credit links cannot be optional. All I can think of is that they needed more designs from the start and designers are more cranky about GPL etc so to accommodate them themes were given the right to have credit links.

So I go back to my original question, as a plugin or theme author, how much credit do you really need?
As much as the end user deem appropriate.

But the more interesting question is: Why should themes be allowed have non optional credit links?
Jeff and others what are your best arguments?