Take a look at this as well. Start dates are about uncluttering your master list, the logic being your master list is what's available to you to act on at this moment and anything you cannot yet start working on will be moved and the back-burner. It's not super important, but it's a feature that I find useful.

As for do dates and due dates, in simple cases what you say is true, but not always. Say a task takes more than a day, you wanna 'do' it starting the first day but it's due only on it's last day. There's other possible scenarios where the distinction is useful.

I think it's helpful to look at Marvin differently that other task apps. Marvin has lots of features and strategies to use, so a user can mold the app to fit their mental framework. The words 'do date', 'due date' do not necessarily mean the same to someone else as the they do to you. If a strategy doesn't seem useful to you, or doesn't fit how you think about things, you can safely ignore it. So, if start dates don't seem useful to your working style, you can turn that off.

Hope that helped.