I agree, however, in general it's best to own the points one needs for
DVC including off years such as cruising. I'd rent over using points ongoing for such options and certainly over buying retail to do so but the best is to not use points or need to rent. IF the cruise periodically meant they'd go to WDW only on the 4th year, I wouldn't buy at all under any circumstances.
So my 2 principles would be to own roughly the points needed with a small buffer of 10-20% (amount depending) but not to go too small. IF not going too small meant they had to rent once every few years, I'd do that over going small unless one found a contract that was small AND esp advantageous and they were very sure of the long terms needs. The OP said every other to every third year which is about the limit of the situation where it's reasonable to considering buying. Usually every other year is reasonable if the other appropriate assumptions apply but every third year is not. A good fit non DVC timeshare and either staying off property or renting for DVC trips is likely the best single option for the situation described.