Personally I think the ban should be a maximum of a month as it is open to abuse or other possibilities, such as the following that come to mind:
For someone that really does not want to have anything to do with Whatsapp - Just install the unofficial client and let the ban take effect, no worry about "accidentally" using it in future.
Play a nasty prank on someone, e.g. ex. partner.Â E.g. pretend to be telling a new partner about all the wonderful features the '+' version has to offer and that you must get it, but e-mailed to the ex by "mistake".Â Whoops...
Phone number recycling - If someone cancels their phone contract (e.g. moving country), generally the phone number gets recycled to a new person over time.Â If that number is banned on Whatsapp, the new number owner will also be banned.
Possible mass abandoning of network - If for someone reason people want to leave Whatsapp in disgust over something and this goes viral, they could install the unofficial app in disgust to get banned to show that they don't to ever use it again.Â