While I think this approach would largely be a bad idea for an IDLE-sized application, where I think it definitely would be useful is in something very restricted like the tkSimpleDialog code. 

To improve the various little 'askstring', 'askinteger' etc. sprinkled throughout IDLE I had to do a copy/paste to change that code to use ttk widgets. Because that is an example of something very simple and self-contained, it could easily provide an option to do ttk or non-ttk widgets. 

Apologies if this is too off-topic.