Putting your work in the calendar enables you to better determine whether or not you can (or should) say yes to a new project. And if you do say yes, you can better determine when you realistically might be able to get it done.
On the flip side, super productive guy, Justin Blanton:
These days I throw everything into OmniFocus: projects/tasks (obviously), birthdays, anniversaries, meetings, etc. Why split any of this stuff across two different apps (if you can get away with using just one)?
I think both are getting at the same point: everything needs to be in one view so you get a better idea of what you actually have to do. This is one of the reasons why the OmniFocus Forecast view (showing your tasks and calendars) is so powerful. Getting to see both at once is extremely helpful, but to date no app has done a very good job at this (even OmniFocus’ attempt is limited).
I have tried both methods mentioned above, the timeboxing-ish method that Markovitz is arguing for is stifling from a creative standpoint for me. Whereas Blanton’s method worked a little bit better for me, I still found it feeling a bit like the wild-wild-west. (Though I will say I love keeping birthdays and anniversaries in OmniFocus, that’s an excellent use.)
The problem that both are trying to solve is finding an intelligent and workable way to tell you one thing: how busy am I, and can I do this too? I wish I could say there was a software solution, or a workflow solution to this, but as far as I have found there isn’t. Instead I just consistently say I am too busy, and hope for the best.
Every solution I have come up with adds a massive amount of “upfront” burden on the workflow. Meaning you need contexts, projects, start dates, due dates, reminders, and time estimates in order to get a good sense for how busy you are. And it hinges on your time estimates being accurate. I suck at being accurate on most time estimates.