Taking a leaf off Apple's approach, you can have single/main iTunes/Apple ID for purchases, and add other Apple ID for personalisation.
However with Windows Phone, the primary ID is used not just purchasing BUT also for many Microsoft services. Which lead to, single household with multiple WP devices and multiple purchases of same applications.
A workaround is to use a separate Microsoft ID as primary ID which controls the purchasing, however no other services will be active on this account. BUT that lead to less-than-ideal approach for not making the most of WP7/7.5's features like integrated SkyDrive or XBoxLive as these depended on the use of primary ID.
Unless this is changing - and happy for anyone who know the insight to this - I don't see WP being all that open/friendly to my way of life... Which is sad as the device and OS are good and with high potential, but just the few niggy limitation that make it less than ideal for us.
Anyone household with WP devices, how do you manage your devices and apps?
p/s: I do know there's XBoxLive Family Pack, if only WP could adopt Family Pack approach yet allowing individualisation for SkyDrive and XBoxLive features, then that'll be awesome. All about cost saving and remove duplication.