I agree that it would be incredibly helpful for linear cueing, but one of the reasons we haven't gone in the direction of trying to emulate timeline-based software (which relies entirely on linear cueing) is because it would introduce a number of complexities (and some impossiblities) due to Isadora's focus on interactive cueing (e.g. video intensity or a transition being controlled by volume using a Sound Level Watcher or by sensors) and non-sequential cueing (e.g. A scene that's programmed to cycle through random Snapshots or an Activate Scene actor in scene 4 activating a background scene 2 with Sound or Movie Players) which simply cannot be tied into or accurately represented on, as you put it, "a timeline sequencer built directly into the interface". This timeline-based (and linear) engine would have to simultaneously exist alongside Isadora's current engine (where non-linear and interactive cueing is possible) and this gets extremely complicated very quickly.
That's not to say this isn't a great idea, it's a wonderful idea. The issue is that from a programming and User Interface/Experience design perspective, the thought, energy, and time required to create, test, and add a timeline functionality as one of the core mechanics of Isadora would be massive. I'm not saying that we can't or won't add this feature, (in fact I hope we do), but I can tell you that unfortunately, we can't implement it anytime soon, since we'd have to scrap a large number of the features that we're planning to implement in the near future in order to focus on incorporating a timeline into the heart of the program.
Best wishes,
Woland