Agreed. For live subjects that can run away, longer the working distance the better. Sigma 70mm minimal focusing distance is 25.7 cm while Canon 100L MFD is 30.48 cm, so it's not a huge benefit. Moving to Canon 180 mm f/3.5 gives you a nice 48 cm but at the cost of higher price and loss of f/stop which some may want for portrait work.
I also try to avoid TC for macro work, as the TC degradations are magnified for macro shots, but sometimes one will have no choice..
The Sigma 70mm is nice because of a very nice bokeh with its 9-blades, ability to function as a walkaround lens on full-frame, while working as nice portrait lens on crop body.