[whpalmer4]

You can have your cake and eat it, too. Here's how.

Launch OmniFocus. Select File->Export... and save a copy of your database in a suitable place, choosing the Backup Document (OmniFocus) format.

Now, delete anything in your current database you don't want. You can use the All Items perspective to show you everything.

At any point in the future where you want to refer to your old data, go to the Finder and double-click on the copy of the database you made in the first step. It will open in another OmniFocus window, and if you decide you need a project or some tasks back, you can just drag them from that window to your current database window.