the reason the Barkers may prefer them is because they use ambient temps for the majority of their heating and that is controled by t stats. Most snake owners dont have climate controled snake rooms so rheostats could vary all over the place. I have use 5 t-stats (2 herpstat pros and 3 Rancos) and have never had a problem whatsoever. Now the cheaper pet store t-stats are horrific.
I would never use a rheostat myself and a Ranco is much cheaper than treating a sick BP.