I'm making some general assumptions here (since you're asking the question) - I think cloning a system (unless you intend to change out the HD) is a little overkill. This is assuming you're going to be using TM.
For me, I have a TC that backs-up my Mini and MBP. I then have an external that I use once month to back-up important files (music, movie, pictures, etc.). I'm currently looking into off-site back-up options for my important files.
Redundancy is key, but it is key on important files. I personally could care less if my computer crashes and I have to re-download Transmit - oh the humanity! 😀