I think it would be terrific if plugin authors could add links onto the front-end to take a little extra credit for their work, but it just isn't possible without turning everyone sites into a big plugin-developer billboard.

You can only run one theme, so adding a credit link isn't going to get in the way. End-users can install dozens of plugins; if each one added it's own link things would just get ridiculous.

I see little point in worrying about such things though. Plugins which add links on the front-end will naturally die out of existence.

IIRC Twitter Tools has an option to add a link on the front-end. I assume it isn't turned on by default (I can't remember), but even if it was, I'd be okay with that since it's very easy to turn off anyway.