I know you hate this idea from a developer perspective, but it's one reason I've held back from buying some software before. I personally use one Mac at a time, but sometimes it's a different Mac.
This is how I sell my software. If one person owns multiple machines that only they themselves use, go ahead and install the same license on each of the machines. I don't think I'd consider tying a piece of software to a specific machine. Too restrictive and too much of a pain in the ass to administer. Plus I personally don't think it makes any sense - it's something I'm opposed to.
If a software package offers a "Family" license for not much more, I go that route
I also offer a family license pack which provides three unique licenses for three people in the same household. That covers the case of multiple people using the product on multiple machines at the same time.
It's possible that the increased exposure of the MAS wold offset any extra income from family licenses.
Personally, I think I'd like to support the MAS while still retailing a version outside the store. I'm customer oriented so I don't want to put anyone out, especially existing customers.
I also do volume sales for corporate and educational customers, either as individual licenses or group licenses. A great portion of my business this past year was in volume sales in fact. This is something developers need to be conscious of - I don't believe the MAS has any provisions for such installations/sales.
With the current software I sell it's unfortunate that I can't have it in the MAS, since it's a preference pane and not a stand-alone app.
I'd still like to see developers keep more than 70%, but again, this is something that can probably be offset by increased exposure and new buying habits. For some developers remarkably so.