If you give TM its own partition, you can limit how much space (and time) it takes up. It won't overwrite anything other than its own oldest backups once it runs out of empty space, either in its own partition or a shared space.
One thing TM is good about that SD & CCC can't address is when you find that files or directories are corrupted, and have been since before your most recent clone/Smart Backup. In those cases, the clone is corrupted, too, and you're hosed. Happened to me in just the last week, both the original and the SD clone were bad, and only TM had the files from various points in the past that I could pull back into the current, and then re-clone.
