I'm sorry for the typos, my hands are acting up. I'm afraid that "Once it hits the Tier 2 or Tier 3, they're not going to bounce it back down to Tier 1 to walk through all the the irrelevant support questions again" was what I meant to say.
I'm a bit surprised you've encountered Tier 2 or Tier 3 who bounce it back. The approach I tend to use, and which I encourage others to use, is that "when it hits Tier 2, bring the Tier 1 staffperson over and train them", or have them help rewrite the scripted responses to get a script that covers the edge cases. And ideally, if it hits Tier 3, they do the same thing with the tier 2 staff.
I'll also note that I'm working with smaller groups than Comcast's customer support center, but the technique works very well for improving the responses, and spreading knowledge downstream and upstream about common problems and their workable solutions. The cross-training is invaluable.