Arguably this violates the Unix philosophy of not spamming the user with non-error messages: http://www.faqs.org/docs/artu/ch11s09.html

• interoperability: it's on standard error and I don't think nix-channel --add has any machine-readable output anyway so it shouldn't break anything in that respect
• vertical screen space: it's a single line
• bandwidth: not really applicable here as it's a practically instantaneous operation

Personally I've found myself forgetting to update channels after modifying them frequently enough that to me this change is justified; @infinisil had the idea after a newbie came asking, having the same issue, on IRC. I won't insist on the change, but personally think it would be for the better.

Unnecessary messages harm composability. For example, a script that does nix-channel --add; nix-channel --update will still print Remember to run nix-channel --update, thereby confusing the user.