OK well that’s an interesting analysis.
My opinion is that yes, it’s generally a bad idea. However, if you’re engaged already there are situations where it is warranted and will turn out OK.
The problems with co-habitation stem more from the reason that one wants to co-habitate in the first place, not the cohabitation itself.
Case-in-point: If a guy shacks up with an easy girl to save cash and get free sex, and she agrees for the same reasons, and at least one of them isn’t of the mind to want to marry, the problems they will have aren’t because they suddenly began living together, but because of the attitudes they already had.
OTOH, a couple that is getting married in 6 months already, moving in together beforehand, is probably going to be just fine, assuming this isn’t something they’ve done seven times before.
My brother shacked up with his fiancee before they got married and they are now divorced and both very bitter about it.
I realize this is just one data point, however, knowing the particular situation the fact that their cohabitation was almost all her idea was a sign of things to come.
I believe that is the point that most of us anti-cohabitationists are trying to make: the relatively small amount of money and hassle that one saves by cohabiting during one's engagement is more than offset by the moral/social/statistical perils.