Myself, I'm going the other way. 80 gigs is way too much for the bootable partition, IMO - I just dedicated 20 GB to a minimal bootable partition, not a full clone, for running utilities such as DiskWarrior. The rest is all for TM.
The advantage of a clone is that it thoroughly backs up all my files and allows me to restore my system back onto the main hard drive if it fails or needs to be reformatted. However, TM already does that, so...