Well, no it is not neciessity, but you WILL have shock mounting problems. By doing the minitub you lose the upper shock mounts. You could weld in a tubular crossmember between the rear frame rails. If you lower your car, by doing that you can run into clearance issues with the rear end and the tubular crossmember. You could also cut holes in the trunk and weld a tubular crossmember in the trunk. There are other options, but it would be much less hassle to just put in the crossmember.
Brian