[Brightcolours]

Dunno the true reasons but that practice didn't survive long, do you see anyone still using this combo, I am assuming it's mostly the presence of better performing affordable native full frame lenses, would you use EFs-17-55f2.8 +TC to have a 24-77f4 or the native 24-105 f4 IS, but again I am  just assuming 

To me it seems you made that practice up. I have not read anywhere about people using a TC to use APS-C lenses on FF in the transition period. 

There were no teleconverters on which you could mount the EF-S 17-55mm f2.8 IS USM, or other EF-S lenses.

 

That said, I see no issue with using my 1.7x (measured more like 1.6x) with my Ultra-Micro-Nikkor 55mm f2 which has too small an image circle on FF, or my modified EF-S 10-18mm f4.5-5.6 IS STM. With the former it crops away the dark corners, and with the latter it gives the lens' full range it was designed for in the 1st place.