I'd like to post my point of view as a cop.
Ofcourse, it'd be better for me as a cop if there were no alliances, many situations would be so easier to deal with. But I can see that a proposal like this would kill 90 percent of the roleplay for the criminal organisations, so I do not support it.
What I would like to be done simular to this, however, is that non-faction or non-allied-faction players wouldnt be able to participate in mafia organised activities. It's kind of irritating when some mafia organises a 207 and random people join just so they can participate in the dm fest.