Ask HN: ticket tracking system vs. collaborative done/todo list?

I've used Bugzilla, Mantis & Trac in previous jobs, but have never seen the huge benefit compared to just keeping a shared online done/todo list (using something like etherpad [1]).

Am I missing something, or do the "featureful" bug tracking system not really help much with a small (<=5) dev team?

[1] http://code.google.com/p/etherpad/