I use this in CGIs pretty often because even testing them throws the warning. If you know your vars will be okay because you're checking them yourself or if it's okay if they're missing, it's harmless.

Also any concat longer than two lines is probably a style mistake. If it's hard to read (by hard to read, I mean hard for the next developer who has to work with it to read), it's not good style and that would be a good segue into one of my current rants with other developers... oh, look at the time.