The comment I just wrote says why, succinctly. It helps if you understand the economics of browser exploit development, and then remind yourself that TBB collapses a whole set of valuable targets down to a single release chain.
Do you have any links you can share to best practices for setting up this secure TOR client instead of using the insecure TBB as explained above?
[1] https://www.torproject.org/download/download-easy.html.en
The TBB has the Tor client and a browser (a slightly tweaked Firefox) configured to connect via the Tor SOCKS proxy rather than via the standard network.
I was disappointed last time I booted up TBB to see they had security by default set to 'Low', which enabled lots of unnecessary stuff, like javascript on for every site by default. Too many content parsers trying to do stuff with untrusted data. Its pretty poor.