Joe Shaw really hit the nail on the head with this post:
If Tracker takes over the world based on its technical merit, that'd be fine. But right now Jamie's attitude really turns me off of his project. Beagle works fine for me, so I have no incentive to switch, anyway.
And since Jamie forgot to propose Tracker as the solution to the Gimmie/BigBoard conflict, one of his fans did it instead:
I find that most people who are looking for alternatives to Beagle (and other Mono apps) make the argument based on politics instead of features. So I guess it's nice that people who refuse to use Mono have choices. But the memory usage argument is less and less relevant with every Beagle or Mono release.