If you are looking to create many-to-many relationships based models in PowerPivot and in BISM Tabular, probably one day you will be interested in using them with Power View (formerly codename “Crescent”). There is a very bad news for you: it appears that, at least in its first release, Power View will not support this scenario, showing a behavior that (and this really worries me) is very different from Excel.
The description of the issue is in this connect item: please vote it! This is going to seriously impact usability of many models from Crescent. I understand the reason of this choice (avoiding selection that doesn’t make sense) but I would like whether there was a way to disable this automatic hiding feature in particular conditions (that might be the routine for a certain data models).
I will investigate for some workaround, but many votes to this issue may raise its priority in Redmond.