For me, it comes down to my personal experience and belief that complex multi-purpose systems are generally less reliable than simple or single-purpose systems. If I ever have to hit the button I don't want to be worrying about the 30% fail rate mentioned above, or that the batteries may be dead due to the tracking feature being left on. The website tracking feature in particular puts the device in "toy" classification in my books since it is not necessary at all for the purposes of calling in SAR during an emergency.
Something else; SPOT is a made in China product, with a 1 year warranty. In comparison to the ACR line (for example) which is made in the US, and comes with a 5 year warranty. Take a look at this: http://www.youtube.com/watch?v=RTcKWGLn8cE
IMO this debate is similar to the smartphone vs dedicated GPS discussion.
But back to the topic at hand, I mentioned the discontinuing of dedicated PLBs because it shows how MEC has really thinned out it's mountain gear lines in favor of things like yoga gear. IMO SPOT is something that appeals more to people who spend a lot of time in the urban environment and want something they can play with, which fits in with MECs trend of catering more and more to customers who rarely get out of the city.