In a lot of cases, I actually see the "Powered by *" link for themes as useful for site visitors. Similarly, I find it somewhat handy when site owners have a "Colophon"-like section that lists the plugins in use on the site (some might see this is a potential security threat, though).

However, if plugins/themes do add any public-facing links, I think it's essential for them to provide an option via the Admin to remove the link. Uncheck a checkbox and link is gone. Unfortunately, most themes have the link hardcoded and force you to edit the theme.