Okay,okay - no such thing as something obvious.

If I just write a script which converts one text file into another, it's overkill.

And certainly this applies for one-liners also. I've never started a one-liner with perl -Te :)

If I write a CGI or something with root rights or generally with foreign user input, it's mandatory.

I use taint mode in any software pertaining to business, and most certainly in scripts which are not user driven, such as glue stuff for critical automation. It doesn't exempt me from looking closely at the modules I use, but it catches dangerous stuff even there.