The issue is a To Do list doesn't live in some isolated universe.
The point is that OF isn't a todo list. OF is merely software that supports your todo system. The entire point of having a todo system is that it is independent of the OS and applications you use. Or in other words: you can have todo's that are tied to people that are not in your contactlist. I wouldn't put some random guy you need to call to retrieve some specific information in your contactlist just because you have to call him once. Also, if you need to call someone that often you probably know the number or you look it up in your contactlist on whatever device you are going to use.
If you are going to tie in with a specific OS you are running the risk of creating an isolated universe. You are dependant on the functionality in that OS. Not just with that application but with your entire todo system. I think that's why OF doesn't use datacollectors. However, OmniGroup is a company that actually listens to their users. If you want to have features like this you can discuss it on their forum or request the feature via their email address.
On a related note, although I can see CalDav calendars in OF calendar view, do you know of a way to actually add items to those CalDav calendars from within OF, or is it only a view into them?
It's for view only. They've added this (you can disable it if you want) to Forecast to give you a better overview because that's what Forecast is for: to give you an overview. It is not meant as a calendaring client. The Forecast needs a bit more work in my opinion (if you are really going to use it as an overview, then one should be able to use it to plan todo's over an entire week; you can't do that atm).
What brought all this up wasn't even work items anyway. I was at a funeral on Wednesday and someone approached me and asked me to call them. I kept forgetting on Thursday and Friday and just wanted to create a quick reminder to call them Monday morning. I also remembered I needed to call my doctor's office about seeing him before the end of the year. I keep getting busy and have forgotten over the last couple of weeks and wanted another quick reminder to call him on Monday as well.
That's what the quick entry and the inbox are for and why one should regularly review the inbox. Siri can be used for quick entry though and it will use Reminders for it (in case of OF it will use a special list which you use with Siri as well).
Funny thing is, todo systems actually require quite some discipline. You need to put stuff in it but you also need to regularly review it and actually do things that you've put in your system. Not very easy I must say (but I'm getting there!).