While acknowledging the value of Two Person Forms, the fact remains that most Tai-Chi Chuan practitioners are hobbyists, who apparently have difficulty finding ample time and motivation to simply practice their Solo Forms enough to acquire much health benefit or martial skill.
Any Two Person Form requires just as much or more time and effort, as well as a regular partner who is also willing to practice with the same quality and consistency as you. It has been my observation that this rarely occurs unless you are able to enlist the participation of a mate or other family member, whom you see daily, and thus have the opportunity to train with on a frequent basis.
In this way, the availability of ready practice partners has always been a prime reason for teaching family members, and has long been a sure method of perpetuating a 'family' style.