If your Time Machine backup contains any mission critical or irreplaceable information, I would use its drive for Time Machine only. Why? To reduce the possibility of non-TM use corrupting or damaging the disk. It’s also good practice to make a second backup of your primary drive on its own disk, using a backup program other than TM.
I maintain backups using Time Machine and Carbon Copy Cloner. The Time Machine drive is always connected. The CCC drive is disconnected most of the time. I do a CCC backup about once a week or before installing an OS update.
I do this for redundancy and to increase the chances of having a clean version of my entire setup in the event of a catastrophic failure or a security breach.
As for free disk space, you might find this article interesting:
ETA: Something I like about CCC is that it is much less opaque than TM. The CCC website has a very comprehensive Help section and knowledge base, the software is updated regularly, and the company founder is very engaged with both the product and its users (for example, he will even post on Mac message boards, see Carbon Copy Cloner restore limitations and Carbon Copy Cloner 6 Problems | MacRumors Forums )